AD704
Rev. E | Page 11 of 16
THEORY OF OPERATION
–V
IN
+V
IN
OUTPUT
+V
S
0.1µF
–V
S
0.1µF
C
t
C2
C1
R3
6.34kΩ
R
G
GAIN TRIM
(500kΩ POT)
R4
47.5kΩ
R5
2.4kΩ
R1
6.34kΩ
R2
49.9kΩ
R6
1MΩ
R10, 2MΩ
C5, 0.01µF
R7
1MΩ
R8
1MΩ
1/4
AD704
1/4
AD704
1/4
AD704
C4
C3
R11, 2MΩ
C6, 0.01µF
R9
1MΩ
1/4
AD704
DC
CMRR
TRIM
(5kΩ POT)
OPTIONAL
AC CMRR TRIM
OPTIONAL BALANCE RESISTOR NETWORKS
CAN BE REPLACED WITH A SHORT.
NOTES
1. INSTRUMENTATION AMPLIFIER GAIN = 1 + + (FOR R1 = R3, R2 = R4 + R5).
2. CAPACITORS C2 AND C4 ARE SOUTHERN ELECTRONICS MPCC, POLYCARBONATE, ±5%, 50V.
3. ALL RESISTORS METAL FILM, 1%.
R2
R1
2R2
R
G
Q
1
=
C1
4C2
ω =
1
R6 C1C2
R6 = R7
Q
2
=
C3
4C4
ω =
1
R8 C3C4
R8 = R9
00818-034
Figure 34. Gain-of-10 Instrumentation Amplifier with Post Filtering
The instrumentation amplifier with post filtering (see Figure 34)
combines two applications that benefit greatly from the AD704.
This circuit achieves low power and dc precision over temperature
with a minimum of components.
The instrumentation amplifier circuit offers many performance
benefits, including BiFET level input bias currents, low input
offset voltage drift, and only 1.2 mA quiescent current. It operates
for gains that are G ≥ 2 and, at lower gains, it benefits from no
output amplifier offset and no noise contribution as encountered
in a 3-op-amp design. Good low frequency CMRR is achieved
even without the optional ac CMRR trim (see Figure 35). Table 4
provides resistance values for three common circuit gains. For
other gains, use the following equations:
5
105)(π2
1
06.0
k8.99
19.0
k9.49
k9.49
×
≈
Ω
=
−
Ω
==
Ω=+=
R3
C
G
RofValueMax
G
R3R1
R5R4R2
t
G
Table 4. Resistance Values for Various Gains
Circuit
Gain (G) R1 and R3
R
G
(Max Value of
Trim Potentiometer)
Bandwidth
(−3 dB), Hz
10 6.34 kΩ 166 kΩ 50 k
100 526 Ω 16.6 kΩ 5k
1000 56.2 Ω 1.66 kΩ 0.5 k
160
140
120
100
80
60
40
20
0
1 10 100 1k 10k
FREQUENCY (Hz)
COMMON-MODE REJECTION (dB)
GAIN = 10, 0.2V p-p COMMON-MODE INPUT
TYPICAL MONOLITHIC IN AMP
WITHOUT CAPACITOR C
t
CIRCUIT TRIMMED
USING CAPACITOR C
t
00818-035
Figure 35. Common-Mode Rejection vs. Frequency with and Without
Capacitor C
t