Datasheet

AD822 Data Sheet
Rev. J | Page 20 of 24
SINGLE-SUPPLY PROGRAMMABLE GAIN
INSTRUMENTATION AMPLIFIER
The AD822 can be configured as a single-supply instrumentation
amplifier that is able to operate from single supplies down to 3 V or
dual supplies up to ±15 V. Using only one AD822 rather than three
separate op amps, this circuit is cost and power efficient. The 2 pA
bias currents of the AD822 FET inputs minimize offset errors
caused by high, unbalanced source impedances.
An array of precision thin film resistors sets the in-amp gain to
be either 10 or 100. These resistors are laser trimmed to ratio
match to 0.01% and have a maximum differential temperature
coefficient of 5 ppm/°C.
Table 6. In-Amp Performance
Parameters V
S
= 3 V, 0 V V
S
= ±5 V
CMRR
74 dB
80 dB
Common-Mode Voltage Range −0.2 V to +2 V −5.2 V to +4 V
3 dB BW
G = 10 180 kHz 180 kHz
G = 100 18 kHz 18 kHz
t
SETTLING
2 V Step 2 µs
5 V Step 5 µs
Noise @ f = 1 kHz
G = 10 270 nV/√Hz 270 nV/√Hz
G = 100 2.2 µV/√Hz 2.2 µV/√Hz
I
SUPPLY
(Total) 1.10 mA 1.15 mA
90
100
10
0%
.
. .
.
. . . .
. .
. ...
. ... .
. . .
. . . .
. . . ... . ... . .
.
. . .
.
. . .
.
. . ...
. ...
. ... .
.
. . . .
. . . ... . ... . .
1V
5µs
00874-048
Figure 48. Pulse Response of In-Amp to a 500 mV p-p Input Signal;
V
S
= 5 V, 0 V; Gain = 0
G = 100
G = 100G = 10 G = 10
1
2
3
4
5
6
7
+
–
+
–
00874-049
OHMTEK
PART # 1043
R5
9kΩ
R4
1kΩ
R3
1kΩ
R2
9kΩ
R1
90kΩ
R6
90kΩ
V
REF
V+
0.1µF
1/2
AD822
1/2
AD822
V
OUT
+
–
+
–
V
IN1
V
IN2
R
P
1kΩ
R
P
1kΩ
(G = 10) V
OUT
= (V
IN1
– V
IN2
) +V
REF
1+
R6
R4 + R5
( )
(G = 100) V
OUT
= (V
IN1
– V
IN2
) +V
REF
R5 + R6
R4
1+
( )
Figure 49. A Single-Supply Programmable Instrumentation Amplifier
LOW DROPOUT BIPOLAR BRIDGE DRIVER
The AD822 can be used for driving a 350 Ω Wheatstone bridge.
Figure 50 shows one half of the AD822 being used to buffer the
AD589, a 1.235 V low power reference. The output of 4.5 V can
be used to drive an analog-to-digital converter (ADC) front end.
The other half of the AD822 is configured as a unity-gain inverter
and generates the other bridge input of −4.5 V. Resistor R1 and
Resistor R2 provide a constant current for bridge excitation. The
AD620 low power instrumentation amplifier is used to condition
the differential output voltage of the bridge. The gain of the AD620
is programmed using an external resistor (R
G
) and determined by
1
k9.49
+
Ω
=
G
R
G
+1.235V
49.9k
Ω
R
1
20Ω
25.4k
Ω 1%
10kΩ 1%
350Ω
350Ω
350
Ω 350Ω
R
G
AD589
10k
Ω 1%
10kΩ
1%
R
2
20Ω
–4.5V
GND
+5V
–5V
1/2
AD822
+
–
+
–
1/2
AD822
8
3
2
1
V+
+V
S
–V
S
V
REF
V+
V–
V–
+
–
AD620
+
–
2
3
4
5
6
7
0.1μF
0.1
μF
1μF
1μF
+ +
+
+
6
5
7
4
TO
A/D CONVERTER
REFERENCE INPUT
00874-051
Figure 50. Low Dropout Bipolar Bridge Driver