Linear Technology DC1070 User manual

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DESCRIPTION
A DC1070 demonstration circuit features the LTC6915
IC, a programmab e gain instrumentation amp ifier.
The LTC® 6915 is a precision programmab e gain in-
strumentation amp ifier. The gain can be programmed
to 0, 1, 2, 4, 8, 16, 32, 64, 128, 256, 512, 1024, 2048,
or 4096 through a para e or seria interface. The offset
is be ow 10  V with a temperature drift of ess than
50nV/°C. The LTC6915 uses charge ba anced samp ed
data techniques to convert a differentia input vo tage
into a sing e ended signa that is in turn amp ified by a
zero-drift operationa amp ifier. The LTC6915 can be
used in sing e power supp y app ications as ow as
2.7V, or with dua ±5V supp ies.
The DC1070 contains an LTC6915, an LCD disp ay and
switches and a PIC (micro-contro er). The PIC reads
the settings of the switches, sends a para e or a seria
contro word to the LTC6915 and disp ays the PGA gain
on the LCD disp ay. The LTC6915 on the DC1070 can
be contro ed with an externa para e or seria digita
contro by moving wire jumpers A-F from position 1-2
to 2-3 and connecting six externa digita contro ines
to on board turrets.
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DEMO CIRCUIT DC1070
QUICK START GUIDE
P R O G R A M M A B L E G A IN ,
P R O G R A M M A B L E G A IN , P R O G R A M M A B L E G A IN ,
P R O G R A M M A B L E G A IN ,
IN S T R U M E N TA T
IN S T R U M E N TA T IN S T R U M E N TA T
IN S T R U M E N TA T IO N A M P L IF IE R , L T C 6 9
IO N A M P L IF IE R , L T C 6 9IO N A M P L IF IE R , L T C 6 9
IO N A M P L IF IE R , L T C 6 9 1 5
1 51 5
1 5
Figure1. LTC6915 Block Diagram

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Quick Test Setup:
1. Turn contrast pot c ockwise to view LCD disp ay.
2. Set red s ide switches as shown in Figure 2: SW 1 s ide eft; SW 2 s ide down; SW 3 s ide up; SW 4 s ide up;
SW 15 s ide down; SW 5 s ide down; SW 6 s ide up (Para e M ode).
3. Set sinewave generator for a 1Vp-p, 50Hz sinewave and connect to osci oscope channe 1 and to IN+ of the
DC1070. Use a c ip to c ip jumper to connect AGND to IN- (or connect the JP1 shunt to the AGND position).
4. Set osci oscope for 10ms/Division and 2V/Division and trigger on channe 1.
5. Connect Dua +/-5V supp y.
Figure 2. Quick Test Setup
SW 1
SW 2
SW 3
SW 5
SW 7
SW 8
SW 9
SW 10
SW 6
SW 15
SW 11
SW 12
SW 13
SW
1
1
SW 14
SW 4

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Quick Test Proce ure:
1. Turn on power supp y. The green LED over SW 6 shou d be on.
2. Press the UP push button SW (SW 11) repeated y and the LCD disp ay shou d step thru AV: 1 to Av: 4096 in
powers of two (1, 2, 4, 8, 16, 32, 64, 128, 512, 1024, 2048, and 4096).
Press the DOW N push button SW (SW 12) unti Av: 1. Press the SEND push button SW (SW 13) and then the
STORE push button SW (SW 14). Channe 2 shou d show a 1Vp-p, 50Hz, sinewave ( the STORE push button
SW saves a PGA gain setting so that it is the gain setting when the board is powered-up ). Press the M IN GAIN
push button SW (SW 9) and the LCD disp ay shou d f ash “M IN GAIN SET”.
3. Press the UP push button SW unti AV: 4. Press the M AX GAIN push button SW (SW 10) and the LCD disp ay
shou d f ash “M AX GAIN SET”.
4. Press and ho d the STEP UP push button SW (SW 7) for about three seconds and channe 2 shou d show a
4Vp-p, 50Hz, sinewave (M AX GAIN setting).
5. Press and ho d the STEP DOW N push button SW (SW 8) for about three seconds and channe 2 shou d show a
1Vp-p, 50Hz, sinewave (M IN GAIN setting).
6. Set SW 5 up (HOLD). The green LED over SW 5 shou d be on and the gain setting can not be changed with any
other switch (SW 7-14).
7. Set SW 5 down (THRU).
8. Set SW 6 down (SERIAL). Steps 2-8 can be repeated in Seria M ode.
Note: the LTC6915 -3dB bandwidth is approximate y 400Hz .
________________________________________________________________________________________
Using a Bri ge Sensor w ith a DC1070
Figure 3 shows the DC1070 connections using a bridge sensor as an input signa source. The bridge sensor can
be any sensor that is configured as a W heatstone resitive bridge with one two or four sensor e ements. Figure 4
shows a resistive bridge that can be used to simu ate the output of a W heatstone bridge sensor.
Test Proce ure Using the Figure 4 Bri ge
1. On DC1070 set SW 3 to COM and SW 15 to EXT and Connect Figure 4 bridge to DC1070 as per Figure 3.
2. Connect bridge supp y, DC1070 power supp y, vo tage source and 6 1/2 DM M as per Figure 3.
3. Set the bridge supp y and the DC1070 power supp y to 3.0V and the vo tage source connected to EXT
VREF to 1.5V. Note: The externa reference vo tage sets the output DC reference (VREF). The bridge input
is equa to [PGA OUT-VREF]/(PGA GAIN). In a sing e supp y operation, if the bridge input is positive
(IN+>IN-) then VREF can be 0V. If the bridge input is ±Vo ts then VREF shou d be at east equa to the
maximum bridge input times the PGA gain. For examp e if the maximum bridge input is ±10mV and the PGA
gain is set to 128 then VREF shou d ≥ 128 X 10mV or 1.28V. W ith a 3V LTC6915 and a VREF equa to 1.5V,
the maximum [PGA OUT-VREF] range is ±1.5V (R oad ≥ 10k).
4. Set the PGA gain to 1 using the UP and DOW N push button SW and adjust the bridge 100 ohm potentiometer
unti [PGA OUT-VREF]=10mV.
5. M easure the bridge vo tage [VB+OUT - VB-OUT] (this is the vo tage of the bridge supp y). The ratio of [PGA
OUT-VREF]/ [VB+OUT - VB-OUT] is a measure of the bridge unba ance or sensor sensitivity. For examp e, if
the bridge sensor is measuring weight in bs and for each b [PGA OUT-VREF]=10mV then [PGA OUT-VREF]/
[VB+OUT - VB-OUT]=0.003333 and the sensor output is 0.003333/b. The ratiometric bridge measurement
provides for bridge sensor ca ibration in units of a physica variab e (weight, pressure, temperature,… ) and is
insensitive to the abso ute va ue of the bridge vo tage or resistance (if the bridge vo tage and resistance is sta-
b e during a measurement).

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Figure 3. Test Setup Using a Bri ge Se
n
sor
F
igure 4. A W heatstone Bri ge

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Using a DC1070 w ith External Control an M easurem ent
Figure 5 shows the DC1070 connections for externa digita contro and measurement. The digita contro ines
can be seria or para e (the LTC6915 data sheet defines the digita contro ine functions and vo tage eve s).
Test Setup for External Control an M easurem ant
1. The DC1070 is provided with wire jumpers A-F in position 1-2 and for externa contro the wire jumpers
must be in position 2-3.
2. Set SW 4 on DC1070 to D-OFF (the power to the DC1070 PIC and LCD disp ay is disconected).
3. Connect a bridge sensor, supp ies, externa vo tage source and ratiometric ADC as shown in Figure 5.
4. Using the Figure 5 setup and the the test procedure for Figure 3 setup as a guide, a comp ete system of
digita contro and measurement for a bridge sensor can be eva uated.
Figure 5. Test Setup Using External Digital Control an M easur
e
m ent

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