AD623

–11–

REV. C

Figure 38. Gain Nonlinearity (G = –100, 15 ppm/Div)

V–

0

0.5

1

(V–) +0.5

(V+) –1.5

V+

(V+) –1.5

(V+) –0.5

1.5

OUTPUT CURRENT – mA

2

Figure 39. Output Voltage Swing vs. Output Current

THEORY OF OPERATION

The AD623 is an instrumentation amplifier based on a modified

classic three op amp approach, to assure single or dual supply

operation even at common-mode voltages at the negative supply

rail. Low voltage offsets, input and output, as well as absolute

gain accuracy, and one external resistor to set the gain, make the

AD623 one of the most versatile instrumentation amplifiers in

its class.

The input signal is applied to PNP transistors acting as voltage

buffers and providing a common-mode signal to the input

amplifiers (Figure 40). An absolute value 50 k

Ω resistor in each

of the amplifiers’ feedback assures gain programmability.

The differential output is

100 kΩ









The differential voltage is then converted to a single-ended

voltage using the output amplifier, which also rejects any common-

mode signal at the output of the input amplifiers.

Since all the amplifiers can swing to either supply rails, as well

as have their common-mode range extended to below the nega-

tive supply rail, the range over which the AD623 can operate is

further enhanced (Figures 19 and 20).

The output voltage at Pin 6 is measured with respect to the

potential at Pin 5. The impedance of the reference pin is 100 k

Ω,

so in applications requiring V/I conversion, a small resistor

between Pins 5 and 6 is all that is needed.

50k

50k

50k

POS SUPPLY

7

INVERTING

2

1

4

50k

50k

50k

8

4

NEG SUPPLY

NON-

INVERTING

3

7

GAIN

OUT

6

REF

5

+

–

+

–

Figure 40. Simplified Schematic

The bandwidth of the AD623 is reduced as the gain is increased,

since all the amplifiers are of voltage feedback type. At unity

gain, it is the output amplifier that limits the bandwidth. There-

fore even at higher gains the AD623 bandwidth does not roll off

as quickly.

APPLICATIONS

Basic Connection

Figure 41 shows the basic connection circuit for the AD623.

should be capacitively decoupled close to the devices power

pins. For best results, use surface mount 0.1

µF ceramic chip

capacitors and 10

µF electrolytic tantalum capacitors.

The input voltage, which can be either single-ended (tie either

–IN or +IN to ground) or differential is amplified by the pro-

grammed gain. The output signal appears as the voltage difference

between the Output pin and the externally applied voltage on

the REF input. For a ground referenced output, REF should be

grounded.

GAIN SELECTION

cisely, by whatever impedance appears between Pins 1 and 8.

The AD623 is designed to offer accurate gains using 0.1%–1%

by using the formula

REFERENCE TERMINAL

The reference terminal potential defines the zero output voltage

and is especially useful when the load does not share a precise

ground with the rest of the system. It provides a direct means of

injecting a precise offset to the output. The reference terminal is

also useful when bipolar signals are being amplified as it can be

used to provide a virtual ground voltage. The voltage on the