AD626
+INPUT
–INPUT
necessary to minimize gain error. Also, any mismatch between the
total source resistance at each input will affect gain accuracy and
common-mode rejection (CMR). For example: when operating at
a gain of 10, an 80 ⍀ mismatch in the source resistance between
the inputs will degrade CMR to 68 dB.
200k⍀
200k⍀
+IN
–IN
1
2
3
8
7
6
5
1/6
ANALOG
GND
G = 100
The output buffer, A2, operates at a gain of 2 or 20, thus setting
the overall, precalibrated gain of the AD626 (with no external
components) at 10 or 100.The gain is set by the feedback network
around amplifier A2.
G = 30
–V
–V
S
+V
S
+V
S
S
0.1F
100k⍀
FILTER
0.1F
OUTPUT
OUT
The output of amplifier A2 relies on a 10 k⍀ resistor to –VS for
“pull-down.” For single-supply operation, (–VS = “GND”), A2
can drive a 10 k⍀ ground referenced load to at least +4.7V.The
minimum, nominally “zero,” output voltage will be 30 mV. For
dual-supply operation ( 5V), the positive output voltage swing
will be the same as for a single supply.The negative swing will be
to –2.5V, at G = 100, limited by the ratio:
4
G = 2
AD626
Figure 6. AD626 Configured for a Gain of 100
+INPUT
R15 + R14
–VS ×
200k⍀
200k⍀
+IN
–IN
–INPUT
1
2
3
4
8
7
6
5
R13 + R14 + R15
1/6
R
H
ANALOG
GND
The negative range can be extended to –3.3V (G = 100) and –4V
(G = 10) by adding an external 10 k⍀ pull-down from the output
to –VS. This will add 0.5 mA to the AD626’s quiescent current,
bringing the total to 2 mA.
G = 100
R
G
G = 30
–V
+V
–V
S
+V
S
S
S
100k⍀
FILTER
0.1F
0.1F
The AD626’s 100 kHz bandwidth at G = 10 and 100 (a 10 MHz
gain bandwidth) is much higher than can be obtained with low
power op amps in discrete differential amplifier circuits. Further-
more, the AD626 is stable driving capacitive loads up to 50 pF
(G10) or 200 pF (G100). Capacitive load drive can be increased
to 200 pF (G10) by connecting a 100 ⍀ resistor in series with the
AD626’s output and the load.
OUT
OUTPUT
G = 2
CF
FILTER
(OPTIONAL)
AD626
1
CORNER FREQUENCY OF FILTER =
2CF (100k⍀)
RESISTOR VALUES FOR GAIN ADJUSTMENT
GAIN RANGE
R
(⍀)
R (⍀)
G
H
ADJUSTINGTHE GAIN OFTHE AD626
4.99k
802
80
11 – 20
20 – 40
40 – 80
80 – 100
100k
10k
1k
The AD626 is easily configured for gains of 10 or 100. Figure 5
shows that for a gain of 10, Pin 7 is simply left unconnected; simi-
larly, for a gain of 100, Pin 7 is grounded, as shown in Figure 6.
2
100
Gains between 10 and 100 are easily set by connecting a variable
resistance between Pin 7 and Analog GND, as shown in Figure 7.
Because the on-chip resistors have an absolute tolerance of 20%
(although they are ratio matched to within 0.1%), at least a 20%
adjustment range must be provided.The values shown in the
table in Figure 7 provide a good trade-off between gain set range
and resolution, for gains from 11 to 90.
Figure 7. Recommended Circuit for Gain Adjustment
SINGLE-POLE LOW-PASS FILTERING
A low-pass filter can be easily implemented by using the features
provided by the AD626.
By simply connecting a capacitor between Pin 4 and ground,
a single-pole low-pass filter is created, as shown in Figure 8.
+INPUT
+INPUT
200k⍀
200k⍀
–IN
+IN
–INPUT
1
2
3
4
8
7
6
5
200k⍀
200k⍀
+IN
–IN
–INPUT
1
2
3
4
8
7
6
5
1/6
NOT
1/6
ANALOG
GND
G = 10
CONNECTED
ANALOG
GND
G = 100
G = 30
G = 30
–V
S
+V
S
–V
+V
S
S
+10V
0.1F
–V
S
+V
S
0.1F
100k⍀
0.1F
100k⍀
FILTER
OUT
FILTER
OUTPUT
G = 2
OUT
OUTPUT
G = 2
AD626
CF
AD626
1
Figure 5. AD626 Configured for a Gain of 10
CORNER FREQUENCY OF FILTER =
2CF (100k⍀)
Figure 8. A One-Pole Low-Pass Filter Circuit
Which Operates from a Single +10 V Supply
–10–
REV. D
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