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SPICE Model

The SPICE macro-model for the AD857x amplifier is given in

Listing 1. This model simulates the typical specifications for the

AD857x, and it can be downloaded from the Analog Devices

website at http://www.analog.com. The schematic of the

macro-model is shown in Figure 61.

Transistors M1 through M4 simulate the rail-to-rail input differ-

ential pairs in the AD857x amplifier. The EOS voltage source in

series with the noninverting input establishes not only the 1

µV

offset voltage, but is also used to establish common-mode and

power supply rejection ratios and input voltage noise. The differ-

ential voltages from nodes 14 to 16 and nodes 17 to 18 are reflected

to E1, which is used to simulate a secondary pole-zero combination

in the open-loop gain of the amplifier.

The voltage at node 32 is then reflected to G1, which adds an

additional gain stage and, in conjunction with CF, establishes

the slew rate of the model at 0.5 V/

µs. M5 and M6 are in a

common-source configuration, similar to the output stage of

the AD857x amplifier. EG1 and EG2 fix the quiescent current

in these two transistors at 100

µA, and also help accurately

simulate the VOUT vs. IOUT characteristic of the amplifier.

The network around ECM1 creates the common-mode voltage

error, with CCM1 setting the corner frequency for the CMRR

roll-off. The power supply rejection error is created by the network

around EPS1, with CPS3 establishing the corner frequency for

the PSRR roll-off. The two current loops around nodes 80 and

81 are used to create a 51 nV/

√Hz noise figure across RN2. All

three of these error sources are reflected to the input of the op

amp model through EOS. Finally, GSY is used to accurately

model the supply current versus supply voltage increase in

the AD857x.

This macro-model has been designed to accurately simulate a

number of specifications exhibited by the AD857x amplifier,

and is one of the most true-to-life macro-models available for

any op amp. It is optimized for operation at 27

°C. Although the

model will function at different temperatures, it may lose accuracy

with respect to the actual behavior of the AD857x.

RC7

RC8

RC3

RC4

EOS

RC2

RC1

RC6

RC5

EREF

CCM1

RCM1

RCM2

ECM1

RN2

RN1

VN1

CPS3

RPS3

RPS4

EPS1

CPS1

RPS1

RPS2

CPS2

GSY

EG1

EVP

EVN

EG2

Figure 61. Schematic of the AD857x SPICE Macro-Model

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