AD625

REV. D

–8–

THEORY OF OPERATION

The AD625 is a monolithic instrumentation amplifier based on

a modification of the classic three-op-amp approach. Monolithic

construction and laser-wafer-trimming allow the tight matching

and tracking of circuit components. This insures the high level

of performance inherent in this circuit architecture.

A preamp section (Q1–Q4) provides additional gain to A1 and

A2. Feedback from the outputs of A1 and A2 forces the collec-

tor currents of Q1–Q4 to be constant, thereby, impressing the

times the differential portion of the input voltage. The unity

gain subtracter, A3, removes any common-mode signal from the

the potential at the reference pin.

gains the transconductance increases. This has three important

advantages. First, this approach allows the circuit to achieve a

very high open-loop gain of (3

thus reducing gain related errors. Second, the gain-bandwidth

product, which is determined by C3, C4, and the input trans-

conductance, increases with gain, thereby, optimizing frequency

response. Third, the input voltage noise is reduced to a value

determined by the collector current of the input transistors

(4 nV/

√Hz).

INPUT PROTECTION

Differential input amplifiers frequently encounter input voltages

outside of their linear range of operation. There are two consid-

erations when applying input protection for the AD625; 1) that

continuous input current must be limited to less than 10 mA

and 2) that input voltages must not exceed either supply by

more than one diode drop (approximately 0.6 V @ 25

°C).

Ω in

series with two diode drops (approximately 1.2 V) between the

plus and minus inputs, in either direction. With no external protec-

Ω), the maximum overload

voltage the AD625 can withstand, continuously, is approximately

±2.5 V. Figure 26a shows the external components necessary to

protect the AD625 under all overload conditions at any gain.

+

–

GAIN

DRIVE

GAIN

DRIVE

50 A50 A

C3

C4

A1

A2

10k

10k

50

GAIN

SENSE

GAIN

SENSE

Q1, Q3

Q2, Q4

50

50 A50 A

10k

10k

–IN

+IN

SENSE

REF

Figure 25. Simplified Circuit of the AD625

The diodes to the supplies are only necessary if input voltages

outside of the range of the supplies are encountered. In higher

gain applications where differential voltages are small, back-to-

back Zener diodes and smaller resistors, as shown in Figure

26b, provides adequate protection. Figure 26c shows low cost

FETs with a maximum ON resistance of 300

Ω configured to offer

input protection with minimal degradation to noise, (5.2 nV/

√Hz

compared to normal noise performance of 4 nV/

√Hz).

During differential overload conditions, excess current will flow

through the gain sense lines (Pins 2 and 15). This will have no

effect in fixed gain applications. However, if the AD625 is being

used in an SPGA application with a CMOS multiplexer, this

current should be taken into consideration. The current capa-

bilities of the multiplexer may be the limiting factor in allowable

overflow current. The ON resistance of the switch should be

protection resistance.

AD625

FD333

FD333

FD333

FD333

–IN

+IN

1.4k

1.4k

Figure 26a. Input Protection Circuit

AD625

FD333

FD333

FD333

–IN

+IN

500

FD333

1N5837A

1N5837A

500

Figure 26b. Input Protection Circuit for G > 5

AD625

FD333

–IN

+IN

FD333

FD333

FD333

2k

2N5952

2k

2N5952

Figure 26c. Input Protection Circuit