Measurement Computing 121 User manual

Operating Manual
Measurement Specialties, Inc. www.meas-spec.com [phone]
32 Journey, Suite 150, Aliso Viejo, CA 92656 USA 1 of 19 t&[email protected]
TM
Precision Low-Noise DC Amplifier
Model 121
Measurement Specialties, Inc.
Vibration Sensors Design Center
32 Journey, Suite 150
Aliso Viejo, CA 92656 USA
Tel: [phone]
www.meas-spec.com
[e-mail]

Operating Manual
Measurement Specialties, Inc. www.meas-spec.com [phone]
32 Journey, Suite 150, Aliso Viejo, CA 92656 USA 2 of 19 t&[email protected]m
TM
Warranty
Measurement Specialties, Inc. accelerometers
are warranted during a period of one year from
date of shipment to original purchaser to be free
from defects in material and workmanship. The
liability of Seller under this warranty is limited to
replacing or repairing any instrument or
component thereof which is returned by Buyer,
at his expense, during such period and which
has not been subjected to misuse, neglect,
improper installation, repair, alteration, or
accident. Seller shall have the right to final
determination as to the existence and cause of a
defect. In no event shall Seller be liable for
collateral or consequential damages. This
warrant is in lieu of any other warranty,
expressed, implied, or statutory; and no
agreement extending or modifying it will be
binding upon Seller unless in writing and signed
by a duly authorized officer.
Receiving Inspection
Every Measurement Specialties, Inc.
accelerometer is carefully inspected and is in
perfect working condition at the time of
shipment. Each accelerometer should be
checked as soon as it is received. If the unit is
damaged in any way, or fails to operate, a claim
should immediately be filed with the
transportation company.
Service Concerns
If a Measurement Specialties, Inc. instrument
requires service, first contact the nearest
Measurement Specialties, Inc. representative.
They may be able to solve the problem without
returning the unit to the factory. If it is
determined that factory service is required, call
Customer Service at the regional headquarters
for an RMA number before return.
Returns
All units being returned to the factory require an
RMA (Return Material Authorization) number
before they will be accepted. This number may
be obtained by calling Customer Service at the
regional headquarters with the following
information; model number(s), quantity, serial
number(s), and symptoms of the problem, if
being returned for service. You must include the
original purchase order number if under
warranty.
Recalibration Services
The Vibration Sensors Design Center and its
two manufacturing facilities in China and France
offer factory re-calibration services for Variable
Capacitance, Piezoresistive, Piezoelectric
(including IEPE, ICP etc.) accelerometers and
Electronics. NIST (US), DKD (Germany),
COFRAC (France) traceable calibration services
on sensitivity at 100 Hz (102 or 120 Hz in
Europe) and full frequency sweeps are offered.
Contact the regional headquarters for pricing
information.
Inquiries
Address all inquiries on operation or applications
to your nearest Sales Representative, or to the
Vibration Applications Support as follows:
Global Headquarters
1000 Lucas Way
Hampton, VA 23666, USA
Tel: [phone]
Vibration Sensors Design Center
32 Journey, Suite 150
Aliso Viejo, CA 92656, USA
Tel: [phone]
European Headquarters
Impasse Jeanne BENOZZI
CS 83 163, 31027 Toulouse Cedex 3, France
Tel: [phone]
Asian Headquarters
Measurement Specialties (China), Ltd.
No. 26 Langshan Road
Shenzhen High-Tech Park (North)
Nanshan District, Shenzhen 518057, China
Tel: [phone]

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TM
Table of Contents
1.0 Description
1.1 Key Features
1.2 Safety
2.0 Functional Characteristics
2.1 Excitation Voltage
2.2 Sensor Sensitivity
2.3 Output Scaling
2.4 Gain
2.5 LP Filter
2.6 Auto-Zero
2.7 Shunt Calibration
2.8 Monitoring State
2.9 Fault Indication
3.0 Operation
3.1 Channel Selection
3.2 Function Selection
3.3 Edit and Display
4.0 Wiring Connection
4.1 Simple Hookup for Bridge Type Sensors
4.2 Full Hookup for Bridge Type Sensors
4.3 Hookup for Transducers Requiring Dual Excitation
4.4 CH1, CH2 and CH3 Transducer Connector
5.0 Front Panel
6.0 Rear Panel
6.1 Input and Output Impedances
6.2 Excitation Output Limit
6.3 Power Requirement
7.0 Location of Filter Modules
8.0 Specification
9.0 Dimensions
10.0 Calibration and Repair
11.0 Ordering Information

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1.0 Description
The Model 121 is a 3-Channel DC Differential Amplifier designed to be used with bridge type or
differential output accelerometers, pressure transducers, and load cells. The amplifier offers an
exceptionally low input noise floor of <20μVrms and wide bandwidth, out to 200 kHz, ideal for transient
measurements.
With the Model 121, the user can instantly program gain or output scaling, and activate Auto-Zero
controls for each channel using the front panel push-buttons. Its microprocessor employs a sleep mode
in order to eliminate clock frequencies after setup is complete. Useful features are shunt calibration, 0 to
25Vdc programmable sensor voltage supply, and a selection of user-installed 4-pole Butterworth LP filter
modules.
1.1 Key Features
Three-Channel DC Differential Voltage Amplifier
Programmable Excitation Voltage
200 kHz Bandwidth (-3dB corner)
Gain Range from 0 to 9999
Alternative 4-pole Butterworth LP filter Module
Auto-Zero
Shunt Calibration
AC & DC Monitoring

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1.2 Safety
Read this manual in its entirety before operating the Model 121 DC Amplifier. Read all wiring and power
hookup instructions and understand the requirements prior to using another manufacturer's products with
the Model 121. Insure that any product being interfaced with the Model 121 is wired according to
prevailing local safety and operational standards before operating.
The following symbols and terms may be found on the Model 121 and its manuals and indicate important
information.
When found on the device, this symbol indicates that the operator should refer to the
manual for important instructions on the proper use of this device. When found in the
manual, this symbol indicates that the reader should understand the implications
contained in the text before operating the device.
This symbol indicates that a shock hazard may be present. Read the instruction manual
carefully and insure that the device is wired properly and that all settings have been
checked prior to applying power to the device.
The WARNING label indicates important information that should be heeded for safe and proper
performance of the device.
The CAUTION label is used to indicate that damage to the power supply or equipment connected
to it could occur if directions are not followed. Warranty could be invalidated if the instructions in
this manual are not followed.
Disassembling the Instrument
Turn Input Power Switch OFF before removing power cable from the instrument.
Remove power cable from instrument before disassembling any part of the
instrument.
Grounding
To avoid electrical shock, the power cord protective grounding conductor must be
connected to power ground.
Fuse Replacement
For continued fire protection, replace fuse only with the specific type and rating by
qualified personnel (Reference Manual "Rear Panel" section). Disconnect the
power cord before replacing fuse.

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2.0 Functional Characteristics
The Model 121 can be manually programmed from the front panel by the following means:
“Channel Select” push-button
Three “channel LEDs”
One “Select Function” push-button
Eight “Function LEDs” indicating which function has been selected by the “Select Function”
push-button
Five character LED display to show the state of each function
Five “EDIT” push-buttons to change the display of the Five character LED
Three LEDs indicating if there is any fault in any of the channels.
The unit stores the settings for all of the functions in its internal memory. At power-up it will automatically
restore the settings from the last session.
2.1 Excitation Voltage
This function is used to set the amplitude of the excitation voltage output.
The user can enter four (4) significant digits ranging from 0.000 to 25.00VDC with 30mA maximum
current supply.
The excitation voltage for the sensor also can be set to dual ±12.5VDC with a different connection to the
9-pin Female D-connector.
The excitation will be set to the desired voltage level after the ENTER key (in the middle of the five EDIT
keys) is pressed.
If the unit is short circuited, or overloaded on a single channel, the excitation voltage of the channel will
indicate 0.000. The Fault LED corresponding to the channel will not extinguish until the fault is resolved
and the power on the Model 121 recycled.
2.2 Sensor Sensitivity
This function selection provides for programming the sensitivity of the transducer (in mV/EU).
EU = Engineering Unit, such as g or m/sec^2
The Model 121 uses this information to determine the gain needed to achieve the required output
scaling.
The user can enter four (4) significant digits ranging from 0.001 to 1000.
(Note: Do not set the sensitivity to 0; otherwise the gain value will be infinity.)

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2.3 Output Scaling
This function selection provides for programming the desired scaling of the output (in mV/EU) of the
amplifier.
The user can enter four significant digits ranging from 0.000 to 1000.
The output scaling will be set to the desired value after the ENTER key (in the middle of the five EDIT
keys) is pressed, and the gain will be set to the value determined by the following equation:
Output Scaling
Amplifier Gain = ---------------------------
Sensitivity
2.4 Gain
This function selection provides for programming the desired gain of the amplifier.
The user can enter four significant digits ranging from 0.000 to 9999.
The gain will be set to the desired value after the ENTER key (in the middle of the five EDIT keys) is
pressed, and the output scaling will be set to the value determined by the following equation:
Output Scaling = Amplifier Gain * Sensitivity
2.5 LP Filter
This function selection provides for enabling or disabling the built-in 4-pole Butterworth LP filter.
The LP filter will be turned ON or OFF after the ENTER key (in the middle of the five EDIT keys) is
pressed,
The -3dB corner frequency of the LP filter is defined by the Filter Module installed by the User.
2.6 Auto-Zero
This function selection provides for auto zeroing the DC output voltage.
Auto-Zero Adjustment Range functions when the output of the amplifier is about ±0.1VDC or
±10VDC/gain, whichever one is greater.

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2.7 Shunt Calibration
This function allows for the connection of either an external or internal shunt calibration resistor. The
location of the shunt calibration resistor can be selected by the User with respect to any of the legs of the
transducer under test.
When the left-right EDIT keys are pressed, the following messages will be shown alternately on the LED
display each time the up-down EDIT keys are pressed:
1. "OFF": The shunt calibration resistor is disconnected from the sensor.
2. "EL P+": The external shunt calibration resistor is connected to the Vexc(+).
3. EL P-": The external shunt calibration resistor is connected to the Vexc(-).
4. "S+ P+": The internal shunt calibration resistor (150Kohm) connected in parallel between the Vexc(+)
and Sig(+) leg of the transducer
5. "S+ P-": The internal shunt calibration resistor (150Kohm) is connected in parallel between the Vexc(+)
and Sig(-) leg of the transducer
6. "S- P+": The internal shunt calibration resistor (150Kohm) is connected in parallel between the Vexc(-)
and Sig(+) leg of the transducer
7. "S- P-": The internal shunt calibration resistor (150Kohm) is connected in parallel between the Vexc(-)
and Sig(-) leg of the transducer
2.8 Monitoring State
This function selection provides for programming the desired state of the Model 121.
The following messages will be shown alternately on the LED display each time the up-down EDIT keys
are pressed: (1) “OFF”, (2) “AC”, and (3) “DC”.
1. "AC": RMS voltage at Vout is monitored and shown on the LED display at the rate of approximately
two (2) times per second. The values displayed on the LED display range from 0.000 to 10.00VAC.
2. "DC": DC voltage at Vout is monitored and shown on the LED display at the rate of approximately two
(2) times per second. The values displayed on the LED display range from -10.00 to 10.00VDC.
3. "OFF": The LED display will be turned OFF. This setting provides the lowest noise performance.
2.9 Fault Indication
There are three (3) fault indicators (Red LEDs) for each channel located on the front panel. The Fault
LED illuminates when the unit encounters a short circuit or an overload condition on a single channel.
The excitation voltage of the channel will then set to 0.000.
Or, if the AC power input voltage is out of range (90~120VAC or 200~250VAC), then all of the Fault
LEDs will not extinguish until the fault is corrected and the power recycled on the Model 121.
The excitation voltage of all three channels will then set to 0.000V.

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3.0 Operation
3.1 Channel Selection
Depress the “Channel Select” push-button to select the desired channel for programming.
Each press of the “Channel Select” push-button will alternately illuminate one of the 3 “Channel LEDs”
located to the right of the “Channel Select” push-button indicating the channel number selected for
programming.
3.2 Function Selection
The user selects which function is being programmed by pressing the “Select Function” push-button.
Each press of the “Select Function” push-button will alternately illuminate one of the “Function LEDs”
indicating the function is selected for programming.
The selected Function LED will illuminate and the current setting will be displayed on the LED display.
Each additional press of the “Select Function” push-button selects the next function and the
corresponding setting is displayed on the LED display.
The following functions can be selected for programming:
(1) Excitation Voltage (2) Sensor Sensitivity
(3) Output Scaling (4) Gain
(5) LP Filter (6) Auto-Zero
(7) Shunt Calibration (8) Monitoring State
When the function is selected, the corresponding “Function LED” will illuminate and stay illuminated until
the “Select Function” push-button is pressed again to select the next function.

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3.3 Edit and Display
The five EDIT keys can be used to change the setting of each function, but make sure to press the
ENTER key (in the middle of the five EDIT keys) after changed the setting. If you do not press the
ENTER key (in the middle of the five keys) after changed the setting, these setting will not be active. The
user can select a function, and then use the “Select Channel” push-button to view/edit the current setting
for the selected function for each channel.
The new setting will be implemented into the unit hardware as soon as the ENTER key (in the middle of
the five EDIT keys) has been pressed. The LED display will show the current setting for the selected
function.
Set Excitation Voltage Output for a Single Channel
Depressing the “Select Function” push-button will alternately illuminate one of the “Function
LEDs” indicating the function is selected for programming.
The “Function LEDs” located to the under of the “Select Function” push-button will
alternately illuminate each time the “Select Function” push-button is pressed.
The “Function LED”, located to the right of the “Voltage Exc. (V)”, stays illuminated. This indicates
the “Voltage Exc. (V)” function is selected for programming.
The current setting for the “Voltage Exc. (V)” will be immediately shown on the LED display.
There are five characters in the LED display. The left hand character is reserved for displaying
the sign of the value. The remaining four characters display the User setting.
Each of the four characters can be changed from 0 to 9 sequentially, and the decimal point also
can be moved left or right.
Then the five EDIT keys can be used to change the value of the “Voltage Exc. (V)”.
The left-right EDIT keys select the number position in the LED display, and then the selected
number position will be blinking to indicate the number selected for programming. The decimal
point act to be the last one of the number position.
The up-down EDIT keys can modify the selected number. Each character can be change
from 0 to 9 sequentially, and the decimal point also can be moved left or right accordingly.
The new setting will be implemented into the unit hardware as soon as the ENTER key (in
the middle of the five EDIT keys) has been pressed.

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Set Sensitivity (mV/EU)
Depress the “Select Function” push-button to select the “Sensitivity (mV/EU)” for programming,
and then the corresponding “Function LED” will illuminate and stay illuminated until the “Select
Function” push-button is pressed again to select the next function.
Modifying the value of the “Sensitivity (mV/EU)” is the same as modifying the “Voltage Exc. (V)”
using the five EDIT keys. Note not to set sensitivity to 0, otherwise the gain value will be infinite.
Set Output Scaling (mV/EU)
The operating method is the same as above about the Voltage Exc. (V) and Sensitivity
(mV/EU).
Set Gain
The operating method is also the same as above.
Set LP Filter (kHz)
Depress the “Select Function” push-button to select the “LP Filter (KHz)” for programming, and
then the corresponding “Function LED” will illuminate and stay illuminated until the “Select
Function” push-button is pressed again to select the next function.
The left-right EDIT keys select the number position in the LED display, and then the selected
number position will be blinking to indicate the number selected for programming.
There will be display as “ON” or ”OFF” alternately on the LED display with depress the up-down
EDIT keys each time.
The LP Filter will be enabled or disabled after the ENTER key (in the middle of the five EDIT
keys) is pressed.
The -3dB corner frequency of the LP Filter is defined by Filter Module installed by the User.
Set Auto-Zero
Depress the “Select Function” push-button to select the “Auto-Zero” for programming, and then
the corresponding “Function LED” will illuminate and stay illuminated until the “Select Function”
push-button is pressed again to select the next function.
The left-right EDIT keys select the number position in the LED display, and then the selected
number position will be blinking to indicate the number selected for programming. And the up-
down EDIT keys can modify the selected number.
When the left-right EDIT keys pressed, the following messages will be shown alternately on the
LED display each time the up-down EDIT keys are pressed: (1) “OFF”, and (2) “ON”.
1. "OFF": When the LED display "OFF", and then press the ENTER key (in the middle of the five
EDIT keys), then the unit sets all of the internal DC offset adjustments to neutral.
2. "ON": When the LED display "ON", and then press the ENTER key (in the middle of the five
EDIT keys), the unit sets the DC output voltage to zero to within ±20mVDC. This process will take
several seconds, and then the LED will display "SUCC". If the unit is unable to set the DC output

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voltage to zero within ±100mVDC, the LED will display "FAIL". And you can press the ENTER key
(in the middle of the five EDIT keys) again to start the Auto-Zero process. And you can select
function to monitoring state and change the setting to DC and then press the ENTER key (in the
middle of the five EDIT keys) to view the DC output voltage on the LED display.
The output of the amplifier needs to be within a range of ±0.1VDC or ±10VDC/gain, whichever is
greater, for the Auto-Zero to function. The resulting zero trim correction value is stored in non-
volatile memory until overwritten in any subsequent Auto-Zero operation. Turning “OFF” Auto-
Zero returns to an un-adjusted output state.
Set Shunt Calibration
Depress the “Select Function” push-button to select the “Shunt Calibration” for programming, and
then the corresponding “Function LED” will illuminate and stay illuminated until the “Select
Function” push-button is pressed again to select the next function.
This function selection provides for connecting the external or internal shunt calibration resistors
installed with several modes.
The left-right EDIT keys select the number position in the LED display, and then the selected
number position will be blinking to indicate the number selected for programming. And the up-
down EDIT keys can modify the selected number. The five characters about the LED display will
be divided up to two parts, and the blinking position can be moved alternately in this two parts
each time the left-right EDIT keys are pressed.
And the following messages will be shown alternately on the LED display each time the up-down
EDIT keys are pressed:
On the left of the two parts, there will be shown alternately as “EL”, “S+” or “S-”. And on the other
side of the two parts, there will be shown as “OFF”, “P+” or “P-” alternately.
The setting will be active only after the ENTER key (in the middle of the five EDIT keys) is
pressed.
Set Monitoring State
Depress the “Select Function” push-button to select the “Monitoring State” for programming, and
then the corresponding “Function LED” will illuminate and stay illuminated until the “Select
Function” push-button is pressed again to select the next function.
The left-right EDIT keys select the number position in the LED display, and then the selected
number position will be blinking to indicate the number selected for programming.
There will be display as “AC”, “DC” or ”OFF” alternately on the LED display with depress the up-
down EDIT keys each time.
The setting will be active only after the ENTER key (in the middle of the five EDIT keys) is
pressed, and then LED display will show the RMS AC voltage output, DC voltage output or
disable the monitoring function accordingly.

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4.0 Wiring Connection
4.1 Simple Hookup for Bridge Type Sensors (Alternate Sense leads Wiring)
When the cable between transducer and the 9-pin Female D-connector of the Model 121 is not too long
and the power consumption of the transducer is not too much, you may choose this simple wiring
connection as following:
4.2 Full Hookup for Bridge Type Sensors (Sense leads Wired for optimum performance)
When the cable between transducer and the 9-pin Female D-connector of 121 is too long and the power
consumption of the transducer is also too much, you may choose this wiring connection as following.
With this wiring connection, the excitation voltage on the transducer will be compensated with voltage
feedback control.

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4.3 Hookup for Transducers Requiring Dual Excitation
A differential output transducer requiring ±Dual Excitation and a common ground connection can be
hooked up as follows.
Note that Vexc(+) is half the voltage set by user with respect to pin 9 (virtual ground) and Vexc(-) is half
the voltage below.
4.4 CH1, CH2 and CH3 Transducer Connector
9-pin Female D-connector
Pin 1: +SENS Positive Excitation Sense
Pin 2: VOLT.REF. Reference Voltage Output
Pin 3: SHUNT External Shunt Resistor
Pin 4: -SIG Negative Transducer Output
Pin 5: +SIG Positive Transducer Output
Pin 6: -SENS Negative Excitation Sense
Pin 7: +EXC Positive Excitation Voltage
Pin 8: -EXC Negative Excitation Voltage
Pin 9: GND Guard/Shield (1/2 Vexc)

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5.0 Front Panel
6.0 Rear Panel
Output BNC
Connector
Function LED
Select Function Push-Button
Channel Select Push-Button
Channel LED
Fault LED
Five EDIT Push-Button
Power ON/OFF Switch
Fuse Installation Socket
Fuse Installation Socket
Transducer Connector
AC Power Input
LED Display

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6.1 Input and Output Impedances
Input：1MegaOhm minimum
Output: 10 Ohm typical
6.2 Excitation Output Limits
Current: 30 mA maximum, short circuit protected
6.3 Power Requirement
90~120 or 200~250 VAC 50/60 Hz, which adjusts automatically
7.0 Locations of Filter Modules
Installed Filter
Module CH1
Installed Filter
Module CH2
Installed Filter
Module CH3

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8.0 Specifications

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9.0 Dimensions
Dimensions shown in inches [millimeters].
INTERPRET GEOMETRIC TOLERANCING PER ANSI Y14.5M-1994.

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10.0 Calibration and Repair
A calibration certificate is provided with each model 121 that certifies each instrument against the listed
performance specifications. It is recommended to perform routine calibrations on the amplifier to ensure
accurate operation. The recommended calibration cycle is one year.
Repair of the model 121 amplifier should not be attempted. Should the amplifier not be functioning
correctly, then it should be returned to the factory for further evaluation.
11.0 Ordering Information
Model 121 must be specified in the following manner: 121-X
If X = U, the Model 121 is supplied with NEMA 5-15 USA 3 pin Plug (120V, 60Hz)
If X = E, the Model 121 is supplied with CEE 7/16 Europlug (220V, 50Hz)
If X = A, the Model 121 is supplied with AS-3112 China, Australasia (220V, 50HZ)