Linear Technology LTC6912-1 User manual

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Centennial, CO 80112
DC861A-A
ANALOG DEVICES
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DC861 QUICK START GUIDE
1
DESCRIPTION
DC861A-A or -B demonstration circuit features a dual digitally pro-
grammable gain amplifier (PGA) C, LTC6912-1 (DC861A-A) or
LTC6912-2 (DC861A-B). The LTC® 6912 is a family of dual chan-
nel, low noise, digitally programmable gain amplifiers (PG A) that
are easy to use and occupy very little PC board space. The gains
for both channels are independently programmable using a 3-
wire SP interface to select voltage gains of 0, 1, 2, 5, 10, 20, 50,
and 100V/V (LTC6912-1); and 0, 1, 2, 4, 8, 16, 32, and 64V/V
(LTC6912-2). All gains are inverting. The LTC6912 consists of 2
matched amplifiers with rail-to-rail outputs. O perating from sin-
gle or split supplies from 2.7V to 10.5V total, the LTC6912-X is
offered in tiny SSO P-16 and DFN -12 Packages (the DC861 board
contains an LTC6912 in the SSOP-16).
The demo board highlights the following:
1. Two independent digitally programmable gain amplifiers.
2. A 2:1 Analog M UX with a dual PGA.
3. Daisy chain of two dual PGAs (must use two DC861boards).
4. A 4:1 Analog M UX with quad PGAs (must use two DC861boards).
On board switches and a 4-Digit LED display provide the user with the
means to input PG A settings to a P C (micro-controller). The P C
sends the settings to the PGA using a 3-wire SP interface. Jumpers
on board configure the demonstration circuit as a dual PGA or a 2:1
analog M UX. A flat ribbon cable and a connector are provided for the
daisy-chain connection of two DC861 boards.
The DC861 demonstration circuit is a stand-alone board (computer
control is not required).
DEMO CIRCUIT
861
A
QUICK START GUIDE
DUAL, SPI PROGRAMABLE GAIN AMP
LIFIER R,
LTC6912-1 AND LTC6912-
2
Figure 1. Demonstration Circuit DC861A-A (LTC6912-1). Figure 2. Block Diagram LTC6912-1/2 SSOP -16 Package.

DC861 QUICK START GUIDE
2
Note 1: States 13 14 and 15 (binary 11xx) are not used. Programming a channel to states 8 or higher w ill configure that particular channel
into a low pow er shutdow n state. In addition programming a channel into state 15 (binary 1111) w ill cause that particular channel to draw
up to 20mA of supply current and is not recommended.
(Note 1)
(
Note 1)
Description of the 3-W ire SPI Interface
Gain control of each amplifier is independently programmable
using the 3-wire SP interface (see Figure 3). Logic
levels for
the LTC6912 3-
wire serial interface are TTL/CM O S compatible.
W hen CS/LD is low, the serial data on D N is shifted into an 8-
bit shift-register on the rising edge
of the clock, with the M SB
transferred first. Serial data on
DOUT is shifted out on the
clock’s falling edge. A rising edge on CS/LD will latch the shift-
register’s contents into an 8-bit D-
latch and disable the clock
internally on the C. The upper nibble of the D-
latch (4 most
significant bits), configures the gain for the B-channel ampli-
fier. The lower nibble of the D-
latch (4 least significant bits),
configures the gain for the A-channel ampli
fier. Tables 1 and 2
detail the nominal gains and respective gain codes.
Figure 3.Block Diagram LTC6912-1/2 Serial Interface

DC861 QUICK START GUIDE
3
Figure 4. DC861A Inputs Outputs Jumpers Sw itches and Connectors.
PGA A Input
PGA B Input
PGA A Output
PGA B Output
Jumper J6 . Sets the LTC6912 for a dual or a single power supply operation.
In a single supply operation the AG ND pin of the LTC6912 is connected to V+/2 thru the
J6 jumper and the V- input must be grounded externally. In addition in a single supply
operation the PGA A and B input should have a DC bias equal to V+/2 or be AC coupled.
Jumper J2: Sets the LTC6912 for a dual PGA or a 2:1
analog M UX operation (the D1 LED is in a 2:1 M UX).
SW 1 to D-On: Provides
power to the digital circuits
and 4-digit LED.
SW 1 to D-Off: Turns off
power to the digital circuits
and 4
-
digit LED.
SELECT SW : Selects 1 of 4 digits
(the selected digit is blinking).
Pressing the SELECT SW for more than three seconds changes the
LED display to show the LTC6912 setting in decimal.
UP SW : ncrements selected digit.
DO W N SW : Decrements selected digit.
SEN D SW : Sends SP control to the LTC6912.
STO R E SW : Stores last SP control setting (available on power-up).
the 4-digit LED display
shows the LTC6912 gain
setting in hex. The display
switches to decimal if the
SELECT sw is pressed for
more than three seconds.
Pow er
Supply
Inputs.
SW 2 to OFF: Sets the board for
stand-alone or slave operation.
SW 2 to ON : Sets the board as mas-
ter SP controller in a two DC861
board connection (the D2 LED is on
in daisy chain master operation).
Jumper J4.
Flat ribbon cable
connection to a
DC861 SP master in
daisy chain operation.
Jumper J3.
Flat ribbon cable connection
to a DC861 SP slave in
daisy chain operation.
SW 3 to ENBL:
Power is on for the
LTC6912.
SW 3 to SHD N :
Sets the LTC6912 to a hard-
ware shut down (D4 LED is
on in SHDN).
For normal board operation
set SW 3 to ENBL.

DC861 QUICK START GUIDE
4
Table 3.
Hex
and Decimal
Digits
and PGA Settings.
Hex ( Note 1) LTC6912-1 Gain Decimal LTC6912-2 Gain Decimal
0 (-120dB) ( Note 2) 0 (-120dB) ( Note 2)
-1 -1
-2 -2
-5 -4
-10 -8
-20 -16
-50 -32
-100 -64
0 (-120dB) ( Note 3) 0 (-120dB) ( Note 3)
0 (-120dB) ( Note 3) 0 (-120dB) ( Note 3)
0 (-120dB) ( Note 3) 0 (-120dB) ( Note 3)
0 (-120dB) ( Note 3) 0 (-120dB) ( Note 3)
0
00
0
1
11
1
2
22
2
3
33
3
4
44
4
5
55
5
6
66
6
7
77
7
8
88
8
9
99
9
Α
ΑΑ
Α
β
ββ
β
0
00
0
0
00
0
0
00
01
11
1
02
0202
02
05
0505
05
10
1010
10
20
2020
20
50
5050
50
1Η
1Η1Η
1Η
Η1
Η1Η1
Η1
Η1
Η1Η1
Η1
Η1
Η1Η1
Η1
Η1
Η1Η1
Η1
Note 1: The hex digits C,
D, E and F are not used.
0
00
0
0
00
0
01
0101
01
02
0202
02
04
0404
04
08
0808
08
16
1616
16
32
3232
32
64
6464
64
Η1
Η1Η1
Η1
Η1
Η1Η1
Η1
Η1
Η1Η1
Η1
Η1
Η1Η1
Η1
Note 2: The input is high impedance and
the output is low impedance.
Note 3: The input and output
are high impedance.
Initial Setup Procedure:
The following is an initial set-up guide to the function of the board’s jum pers, switches and connectors (see Figure 4).
A. Dual PGA (one DC861)
1. Connect a DC861 board as shown in Figure 5.
(J2-Dual, J6-DUAL, SW 1-D-ON , SW 2-OFF, SW 3-ENBL).
2. Set the power supply to ±5V and turn it on.
3. Turn on the sinewave generator and set it to a 1Vp-p, 1kHz sinewave
(refer to Tables 1 or 2 for the nominal PGA A and B input impedance).
4. On power up the 4-digit LED display shows the LTC6912 gain settings in hex
(The gain setting shown on power up is the last stored setting).
Holding the SELECT sw for more than three seconds changes the display to show decimal digits
The correspondence between a hex number and a PGA setting is shown on Table 3.
(See Figure 6 for display examples in hex and decimal digits).
6. Use the SELECT and UP and DOW N switches to select the gain and press the SEND switch to send the data to
the LTC6912 (for a quick board test, set the PGA A and B gain to one (0dB) and the oscilloscope channels #1
and #2 connected to OUT A and OUT B respectively should show a 1Vp-p, 1kHz sinewave (1Vp-p input times
one).

DC861 QUICK START GUIDE
5
B. 2:1 Analog M UX
1. Connect a DC861 board as shown in Figure 5.
(J2-Dual, J6-M UX’D, SW 1-D-ON , SW 2-OFF, SW 3-ENBL).
2. Set the power supply to ±5V and turn it on.
3. Turn on the sinewave generator and set it to a 1Vp-p, 1kHz sinewave
(refer to Tables 1 or 2 for the nominal PGA A and B input impedance).
4. Connect one oscilloscope channel to an output (in a M UX operation OUT A and B are shorted).
5. Use the SELECT and UP and DOW N switches to set the gain to one (0dB) for PGA A and two (6dB) for PGA B
and press the SEND switch to send the data to the LTC6912.
6. Pressing the SEND switch repeatedly will show on the LED display an 01 or an 02 (the 2:1 M UX gain switches
from one to two) and the output between OUT A or OUT B (the oscilloscope display should alternately show a
1Vp-p and a 2Vp-p, 1kHz sinewave (the 2:1 M UX switches between amplifier gains one and two).
C. QUAD PGA (Daisy Chain Operation of tw o DC861)
Note In a daisy chain m ode, the PGA settings are set in hex and on SEND the settings are shown in decim al.
1. Connect two DC861 boards as shown in Figure 7.
(jumpers and switches on master and slave boards, J2-Dual, J6-DUAL, SW 1-D-ON, SW 3-ENBL).
2. On the master board set SW 2 to ON and on the slave board set SW 2 to OFF.
3. Set the power supply to ±5V and turn it on.
4. Turn on the sinewave generator and set it to a 0.1V p-p (100mV), 1kHz sinewave
(refer to Tables 1 or 2 for the nominal PGA A and B input impedance).
5. The 4-digit display on the slave board should be blank and the gain settings for all four PGAs shown in hex digits on
the master board’s 4-digit LED display.
6. Use the SELECT and UP and DOW N switches to set on the master’s display the gains in hex of the four PGAs
as follows: #1 digit to 4, #2 digit to 3, #3 digit to 2 and #1 digit to 1.
Note Before the SEND sw is pressed, the m aster display shows the PGA gain from left to right in the same sequence of the
16-bit SPI word (the setting for the slave PGA B is sent first then the setting for the slave PGA A, then the setting for the
m aster PGA B and last the setting for the master PGA A).
7. Press the SEND switch to send the data to the LTC6912 on the master and slave boards.
8. The master and slave display and oscilloscope display should show:
a. M aster display digits #1 and 2, PGA B gain=02 and on oscilloscope channel #2 a 0.2Vp-p, 1kHz sinewave.
b. M aster display digits #3 and 4, PGA A gain=01 and on oscilloscope channel #1 a 0.1Vp-p, 1kHz sinewave.
c. Slave display digits #1 and 2, PGA B gain=10 (LTC6912-1) or PGA B gain=08 (LTC6912-2) and on
oscilloscope channel #4 a 1.0Vp-p, 1kHz sinewave (LTC6912-1) or a 0.8Vp-p, 1kHz sinewave (LTC6912-2).
d. Slave display digits #3 and 4, PGA A gain=05 (LTC6912-1) or PGA B gain=04 (LTC6912-2) and on
oscilloscope channel #3 a 0.5Vp-p, 1kHz sinewave (LTC6912-1) or a 0.5Vp-p, 1kHz sinewave (LTC6912-2).
(See Figure 7 example).

DC861 QUICK START GUIDE
6
D. A 4:1 Analog M UX PG A (Daisy Chain Operation of tw o DC861)
Note In a daisy chain m ode, the PGA settings are set in hex and on SEND the settings are shown in decim al.
1. Connect two DC861 boards as shown in Figure 7.
(jumpers and switches on master and slave boards, J2-Dual, J6-M UX’D, SW 1-D-ON, SW 2-OFF, SW 3-ENBL).
2. On the master board set SW 2 to ON and on the slave board set SW 2 to OFF.
Note The daisy chain flat ribon cable shorts the master board’s OUT A and B to the slave board’s OUT A and B.
3. Set the power supply to ±5V and turn it on.
4. Turn on the sinewave generator and set it to a 0.1V p-p (100mV), 1kH z sinewave
(refer to Tables 1 or 2 for the nominal PGA A and B input impedance).
5. The 4-digit display on the slave board should be blank and the gain settings for all four PGAs shown in hex digits on
the master board’s 4-digit LED display.
6. Use the SELECT and UP and DOW N switches to set on the master’s display the gains in hex of the four PGAs
as follows: #1 digit to 4, #2 digit to 3, and #3 digit to 2 and #1 digit to 1.
7. Press the SEND switch repeatedly and the display will show in sequence:
a. on the master display 01 then 02 then on the slave display 10 and 05 (LTC6912-1) or
b. on the master display 01 then 02 then on the slave display 08 and 04 (LTC6912-2).
The oscilloscope display should show in sequence:
a. 0.1Vp-p, 1kHz sine, 0.2Vp-p, 1kHz sinewave, 1.0Vp-p, 1kHz sinewave and 0.5Vp-p (for an LTC6912-1) or
b. 0.1Vp-p, 1kHz sine, 0.2Vp-p, 1kHz sinewave, 0.8Vp-p, 1kHz sinewave and 0.4Vp-p (for an LTC6912-2).
ADDITIO NAL BO ARD FUNCTIONS
Note A: For the lowest analog noise and distortion tests, set SW 1 to D-OFF after pressing the SEND sw.
W hen SW -1 is set to D-OFF, the board’s digital circuits are disconnected and the LTC6912 is a fixed gain
amplifier (dual or single depending if the J6 junper is in DUAL or M UX’D position). The amplifier’s gain
is the last setting before the SEND sw is pressed.
Note B. For an optional external serial control of the LTC6912, remove the jumper wires of J1 from 1-2, 4-5 and
7-8 and connect jumper wires to 2-3, 5-6 and 8-9 (see DC861 schematic). The external SP control
inputs are the CLK, DN and CS/LD miniature turrets under connector J4 (on the lower left corner of
DC861).
Note C. The VSLV miniature turret under the J4 connector is for monitoring the digital 5V voltage of the slave
board in a daisy chain connection. The +5V miniature turret under the J4 connector is for monitoring
the DC861 5V switching regulator that provides power to the board’s digital circuits.

DC861 QUICK START GUIDE
7
Figure 5. Dual PG A or 2:1 M UX Quick Set-Up (Dual Supply Operation)
Oscilloscope
#1 #2
Connect:
Out A to #1
Oscilloscope nput
Out B to #2
Oscilloscope nput
Sinew ave Generator
Connect generator output
to NA and NB
Dual Pow er Supply
Ground
Lead
Ground
Lead
Figure 6. LED Display Examples in Hex and Decimal
(LTC6912-1 or LTC6912-2)
Note: Only one oscilloscope channel is required
for output monitoring in a 2:1 M UX operation.

DC861 QUICK START GUIDE
8
Connect:
Out A of M aster to #1 Oscilloscope nput
Out B of M aster to #2 Oscilloscope nput
Out A of Slave to #3 Oscilloscope nput
Out B of Slave to #4 Oscilloscope nput
Figure 7. Quad PGA or 4:1 M UX Daisy Chain Quick Set-Up (Dual Supply Operation)
Sinew ave Generator
Connect generator output to NA
and N B of M ASTER board.
Note: Only one oscilloscope channel is required
for output monitoring in a 4:1 M UX operation.