AD708J(RevC) Ver la hoja de datos (PDF) - Analog Devices
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AD708J Datasheet PDF : 16 Pages
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OPERATION WITH A GAIN OF −100
To show the outstanding dc precision of the AD708 in a real
application, Table 3 shows an error budget calculation for a gain
of −100. This configuration is shown in Figure 28.
Table 3.
Error Sources
VOS
IOS
Gain (2 kΩ Load)
Noise
VOS Drift
Total Unadjusted
Error
With Offset
Calibrated Out
Maximum Error Contribution
AV = 100 (S Grade)
(Full Scale: VOUT = 10 V, VIN = 100 mV)
30 μV/100 mV
= 300 ppm
(100 kΩ)(1 nA)/10 V
= 10 ppm
10 V/(5 × 106)/100 mV = 20 ppm
0.35 mV/100 mV
= 4 ppm
(0.3 mV/°C)/100 mV
= 3 ppm/°C
@ 25°C
−55°C to +125°C
= 334 ppm > 11 bits
= 634 ppm > 10 bits
@ 25°C
−55°C to +125°C
= 34 ppm > 14 bits
= 334 ppm > 11 bits
1kΩ
VIN
1kΩ
100kΩ
+VS
0.1µF
2– 7
1/2
AD708 6
3 + 4 0.1µF
VOUT
–VS
Figure 28. Gain of −100 Configuration
This error budget assumes no error in the resistor ratio and no
error from power supply variation (the 120 dB minimum PSRR
of the AD708S makes this a good assumption). The external
resistors can cause gain error from mismatch and drift over
temperature.
HIGH PRECISION PROGRAMMABLE GAIN
AMPLIFIER
The three op amp programmable gain amplifier shown in
Figure 29 takes advantage of the outstanding matching
characteristics of the AD708 to achieve high dc precision.
AD708
1/2
VINA
AD708
OUT
1–4
A0
S1
A1
S2
S3
S4
10kΩ
10kΩ
10kΩ
9.9kΩ
RA
10kΩ
AD7502
–VS
100Ω
1kΩ
10kΩ
10kΩ
AD707
+VS
S8
S7
9.9kΩ
S6
S5
RB
OUT
5–8
10kΩ
10kΩ
10kΩ
VINB
1/2
AD708
Figure 29. Precision PGA
The gains of the circuit are controlled by the select lines, A0 and
A1, of the AD7502 multiplexer, and are 1, 10, 100, and 1000 in
this design.
The input stage attains very high dc precision due to the 30 μV
maximum offset voltage match of the AD708S and the 1 nA
maximum input bias current match. The accuracy is main-
tained over temperature because of the ultralow drift
performance of the AD708.
To achieve 0.1% gain accuracy, along with high common-mode
rejection, the circuit should be trimmed.
To maximize common-mode rejection
1. Set the select lines for gain = 1 and ground VINB.
2. Apply a precision dc voltage to VINA and trim RA until
VO = −VINA to the required precision.
3. Connect VINB to VINA and apply an input voltage equal to
the full-scale common mode expected.
4. Trim RBB until VO = 0 V.
To minimize gain errors
1. Select gain = 10 with the control lines and apply a
differential input voltage.
2. Adjust the 100 Ω potentiometer to VO = 10 VIN
(adjust VIN magnitude as necessary).
3. Repeat Step 1 and Step 2 for gain = 100 and gain = 1000,
adjusting the 1 kΩ and 10 kΩ potentiometers, respectively.
The design shown in Figure 29 should allow for 0.1% gain
accuracy and 0.1 μV/V common-mode rejection when ±1%
resistors and ±5% potentiometers are used.
Rev. C | Page 11 of 16
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