AD626

–9–

10

90

100

0%

AD626

C1

5pF

+IN

–IN

R1

200k

R2

200k

R3

41k

C2

5pF

R4

41k

R5

4.2k

R11

10k

R6

500

R7

500

R8

10k

R10

10k

R14

555

GND

GAIN = 100

R13

10k

R15

10k

R17

95k

R9

10k

R12

100k

FILTER

OUT

A1

A2

Figure 4. Simplified Schematic

10

90

100

0%

10k

10k

10k

INPUT

20V p–p

1k

AD626

ERROR

OUT

2k

Figure 3. Settling Time Test Circuit

THEORY OF OPERATION

The AD626 is a differential amplifier consisting of a precision

balanced attenuator, a very low drift preamplifier (A1), and an

output buffer amplifier (A2). It has been designed so that small

differential signals can be accurately amplified and filtered in the

of any other active components.

Figure 4 shows the main elements of the AD626. The signal inputs

at Pins 1 and 8 are first applied to dual resistive attenuators R1

through R4 whose purpose is to reduce the peak common-mode

voltage at the input to the preamplifier—a feedback stage based

on the very low drift op amp A1. This allows the differential

input voltage to be accurately amplified in the presence of large

common-mode voltages six times greater than that which can be

tolerated by the actual input to A1. As a result, the input CMR

mode error is minimized by precise laser-trimming of R3 and R4,

thus giving the AD626 a common-mode rejection ratio (CMRR)

of at least 10,000:1 (80 dB).

To minimize the effect of spurious RF signals at the inputs due to

rectification at the input to A1, small filter capacitors C1 and C2

are included.

The output of A1 is connected to the input of A2 via a 100 k

(R12) resistor to facilitate the low-pass filtering of the signal of

interest (see Low-Pass Filtering section).

The 200 k

input impedance of the AD626 requires that the source

resistance driving this amplifier be low in value (<1 k )—this is

REV. D