AD625 AD [Analog Devices], AD625 Datasheet - Page 9

• Manufacturer Part Number: AD625
• Description: Programmable Gain Instrumentation Amplifier
• Manufacturer: AD [Analog Devices]

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:
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
Selecting Resistor Values
As previously stated each R
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
RTO errors increases with increasing feedback resistance, values
much above 20 kΩ are not recommended (values below 10 kΩ
for R
offset, drift, and bandwidth (Figure 28) when selecting the
feedback resistors. The gain resistor (R
formula R
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.
F
may lead to instability). Refer to the graph of RTO noise,
G
F
+GAIN DRIVE
= 2 R
.
SENSE
+GAIN
+V
4 kTR
RTI NULL
RTI NULL
R
S
F
REF
–V
NC
F
S
/(G – l).
ext
G =
1
2
3
4
5
6
7
8
+INPUT
+(4 nV/ Hz )
2R
R
G
F
AD625
10k
+1
10k
F
A1
provides feedback to the input
R
A3
G
A2
10k
10k
2
F
–INPUT
. Since the magnitude of
G
= 7.9 nV / Hz
) is determined by the
16
15
14
13
12
11
10
9
–GAIN DRIVE
+V
RTO
NULL
RTO
NULL
S
–GAIN
SENSE
R
V
F
OUT
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
Table I. Common Gains Nominally Within
Using Standard 1% Resistors
1000
1024
GAIN
100
200
500
128
256
512
300
200
100
10
20
50
16
32
64
6
5
4
3
2
1
10k
10k
1
2
5
4
8
RTO OFFSET VOLTAGE DRIFT
FEEDBACK RESISTANCE –
FEEDBACK RESISTANCE –
20k
20k
RTO NOISE
30k
30k
40k
40k
50k
50k
R
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
60k
60k
F
100k
10k
1M
3
2
10k
1
FEEDBACK RESISTANCE –
FEEDBACK RESISTANCE –
RTO OFFSET VOLTAGE
20k
Ω
BANDWIDTH
10
30k
20k
0.5% Error
10k
AD625
40k
50k
100
R
∞
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
50k
G
60k
1k