AD625
REV. D
–9–
Any resistors in series with the inputs of the AD625 will degrade
the noise performance. For this reason the circuit in Figure 26b
should be used if the gains are all greater than 5. For gains less
than 5, either the circuit in Figure 26a or in Figure 26c can be
used. The two 1.4 k
resistors in Figure 26a will degrade the
noise performance to:
4 kTRext
+(4 nV/ Hz)2 = 7.9 nV/ Hz
RESISTOR PROGRAMMABLE GAIN AMPLIFIER
In the resistor-programmed mode (Figure 27), only three exter-
nal resistors are needed to select any gain from 1 to 10,000.
Depending on the application, discrete components or a
pretrimmed network can be used. The gain accuracy and gain
TC are primarily determined by the external resistors since the
AD625C contributes less than 0.02% to gain error and under
5 ppm/
°C gain TC. The gain sense current is insensitive to
common-mode voltage, making the CMRR of the resistor pro-
grammed AD625 independent of the match of the two feedback
resistors, RF.
Selecting Resistor Values
As previously stated each RF provides feedback to the input
stage and sets the unity gain transconductance. These feedback
resistors are provided by the user. The AD625 is tested and
specified with a value of 20 k
for RF. Since the magnitude of
RTO errors increases with increasing feedback resistance, values
much above 20 k
are not recommended (values below 10 k
for RF may lead to instability). Refer to the graph of RTO noise,
offset, drift, and bandwidth (Figure 28) when selecting the
feedback resistors. The gain resistor (RG) is determined by the
formula RG = 2 RF/(G – l).
+GAIN
SENSE
–GAIN
SENSE
+INPUT
–INPUT
RTI NULL
RTO
NULL
RTO
NULL
+VS
+GAIN DRIVE
–GAIN DRIVE
RF
RG
RF
NC
REF
–VS
VOUT
+VS
G =
+1
2RF
RG
A1
A2
AD625
10k
A3
1
2
3
4
5
6
7
8
16
15
14
13
12
11
10
9
Figure 27. AD625 in Fixed Gain Configuration
A list of standard resistors which can be used to set some com-
mon gains is shown in Table I.
For single gain applications, only one offset null adjust is neces-
sary; in these cases the RTI null should be used.
RTO OFFSET VOLTAGE DRIFT
6
5
4
3
2
1
60k
50k
40k
30k
20k
10k
MULTIPLYING
FACTOR
BANDWIDTH
1M
100k
10k
1
10
100
1k
FREQUENCY
–
Hz
10k
20k
50k
FEEDBACK RESISTANCE –
RTO NOISE
RTO OFFSET VOLTAGE
300
200
100
3
2
10k
20k
30k
40k
50k
60k
10k
20k
30k
40k
50k
60k
VOLTAGE
NOISE
–
nV
Hz
MULTIPLYING
FACTOR
FEEDBACK RESISTANCE –
Figure 28. RTO Noise, Offset, Drift and Bandwidth vs.
Feedback Resistance Normalized to 20 k
Table I. Common Gains Nominally Within
0.5% Error
Using Standard 1% Resistors
GAIN
RF
RG
1
20 k
∞
2
19.6 k
39.2 k
5
20 k
10 k
10
20 k
4.42 k
20
20 k
2.1 k
50
19.6 k
806
100
20 k
402
200
20.5 k
205
500
19.6 k
78.7
1000
19.6 k
39.2
4
20 k
13.3 k
8
19.6 k
5.62 k
16
20 k
2.67 k
32
19.6 k
1.27 k
64
20 k
634
128
20 k
316
256
19.6 k
154
512
19.6 k
76.8
1024
19.6 k
38.3
SENSE TERMINAL
The sense terminal is the feedback point for the AD625 output
amplifier. Normally it is connected directly to the output. If
heavy load currents are to be drawn through long leads, voltage
drops through lead resistance can cause errors. In these in-
stances the sense terminal can be wired to the load thus putting