AD626 Просмотр технического описания (PDF) - Analog Devices

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AD626

Low Cost, Single-Supply Differential Amplifier

Analog Devices 

AD626

100

100

90

90

10

0%

TPC 25. Settling Time. VS = +5 V, G = 10

10

0%

TPC 26. SettlingTime. VS = +5 V, G = 100

INPUT

20V p–p

10k⍀

10k⍀

1k⍀

2k⍀

+VS

10k⍀

AD626

ERROR

OUT

–VS

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

presence of large common-mode voltages (VCM), without the use

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

extends to six times the quantity (VS – 1 V). The overall common-

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

+VS

FILTER

C1

R1

5pF

200k⍀

+IN

–IN

R2

200k⍀

R3

41k⍀

A1

C2

5pF

R4

41k⍀

R9

10k⍀

R12

100k⍀

R17

95k⍀

AD626

A2

R15

10k⍀

R11

10k⍀

R6

500⍀

R5

4.2k⍀

R7

500⍀

R8

10k⍀

R10

10k⍀

R14

555⍀

R13

10k⍀

OUT

REV. D

GND

GAIN = 100

–VS

Figure 4. Simplified Schematic

–9–

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