AD625_15 Datasheet, PDF (9/15 Pages) Analog Devices – Programmable Gain Instrumentation Amplifier

AD625
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 kΩ resistors in Figure 26a will degrade the
noise performance to:
4 kTR_ext +(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, R_F.
Selecting Resistor Values
As previously stated each R_F 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 kΩ for R_F. Since the magnitude of
RTO errors increases with increasing feedback resistance, values
much above 20 kΩ are not recommended (values below 10 kΩ
for R_F 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 (R_G) is determined by the
formula R_G = 2 R_F/(G – l).
G
=
2R_F
R_G
+1
R_F
R_G
R_F
+INPUT
–INPUT
1
+GAIN
SENSE
2
RTI NULL
3
+V_S
4
RTI NULL
5
+GAIN DRIVE
NC 6
REF
7
–V_S 8
A1
A2
10k⍀
10k⍀
10k⍀
10k⍀
A3
AD625
16
–GAIN
15
SENSE
RTO
14 _NULL
13
RTO
NULL
12
–GAIN DRIVE
11
V_OUT
10
9 +VS
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 NOISE
RTO OFFSET VOLTAGE
300
3
200
2
100
10k 20k 30k 40k 50k 60k
FEEDBACK RESISTANCE – ⍀
10k 20k 30k 40k 50k 60k
FEEDBACK RESISTANCE – ⍀
RTO OFFSET VOLTAGE DRIFT
6
5
4
3
2
1M
100k
10k
BANDWIDTH
10k⍀
20k⍀
50k⍀
1
10k 20k 30k 40k 50k 60k
FEEDBACK RESISTANCE – ⍀
1
10
100
1k
FEEDBACK RESISTANCE – ⍀
Figure 28. RTO Noise, Offset, Drift and Bandwidth vs.
Feedback Resistance Normalized to 20 kΩ
Table I. Common Gains Nominally Within ؎0.5% Error
Using Standard 1% Resistors
GAIN
1
2
5
10
20
50
100
200
500
1000
4
8
16
32
64
128
256
512
1024
R_F
20 kΩ
19.6 kΩ
20 kΩ
20 kΩ
20 kΩ
19.6 kΩ
20 kΩ
20.5 kΩ
19.6 kΩ
19.6 kΩ
20 kΩ
19.6 kΩ
20 kΩ
19.6 kΩ
20 kΩ
20 kΩ
19.6 kΩ
19.6 kΩ
19.6 kΩ
R_G
∞
39.2 kΩ
10 kΩ
4.42 kΩ
2.1 kΩ
806 Ω
402 Ω
205 Ω
78.7 Ω
39.2 Ω
13.3 kΩ
5.62 kΩ
2.67 kΩ
1.27 kΩ
634 Ω
316 Ω
154 Ω
76.8 Ω
38.3 Ω
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
REV. D
–9–