AD620

Rev. G | Page 13 of 20

THEORY OF OPERATION

A1

A2

A3

C2

R1

R2

GAIN

SENSE

GAIN

SENSE

R3

400

Ω

10k

Ω

10k

Ω

I2

I1

10k

Ω

REF

10k

Ω

+IN

– IN

20

µA

20

µA

R4

400

Ω

OUTPUT

C1

Q2

Q1

Figure 38. Simplified Schematic of AD620

The AD620 is a monolithic instrumentation amplifier based on

a modification of the classic three op amp approach. Absolute

value trimming allows the user to program gain accurately

(to 0.15% at G = 100) with only one resistor. Monolithic

construction and laser wafer trimming allow the tight matching

and tracking of circuit components, thus ensuring the high level

of performance inherent in this circuit.

The input transistors Q1 and Q2 provide a single differential-

pair bipolar input for high precision (Figure 38), yet offer 10×

lower input bias current thanks to Superϐeta processing.

Feedback through the Q1-A1-R1 loop and the Q2-A2-R2 loop

maintains constant collector current of the input devices Q1

and Q2, thereby impressing the input voltage across the external

unity-gain subtractor, A3, removes any common-mode signal,

yielding a single-ended output referred to the REF pin potential.

transconductance increases asymptotically to that of the input

transistors. This has three important advantages: (a) Open-loop

gain is boosted for increasing programmed gain, thus reducing

gain related errors. (b) The gain-bandwidth product

(determined by C1 and C2 and the preamp transconductance)

increases with programmed gain, thus optimizing frequency

response. (c) The input voltage noise is reduced to a value of

9 nV/√Hz, determined mainly by the collector current and base

resistance of the input devices.

The internal gain resistors, R1 and R2, are trimmed to an

absolute value of 24.7 kΩ, allowing the gain to be programmed

accurately with a single external resistor.

The gain equation is then

1

4

.

49

+

Ω

=

G

R

k

G

1

4

.

49

−

Ω

=

G

k

R

G

Make vs. Buy: a Typical Bridge Application Error Budget

The AD620 offers improved performance over “homebrew”

three op amp IA designs, along with smaller size, fewer

components, and 10× lower supply current. In the typical

application, shown in Figure 39, a gain of 100 is required to

amplify a bridge output of 20 mV full-scale over the industrial

temperature range of −40°C to +85°C. Table 3 shows how to

calculate the effect various error sources have on circuit

accuracy.