AD8591/AD8592/AD8594

–12–

REV. A

SPICE Model for the AD8591/AD8592/AD8594 Amplifier

The SPICE model for the AD8591/AD8592/AD8594 amplifier is

one of the more realistic computer simulation macro-models

available, providing a high degree of realism with respect to char-

acteristics of the actual amplifier. This model, shown in Listing 1,

is based on typical values for the device and can be downloaded

from Analog Devices’ Internet site at www.analog.com.

The model uses a common source output stage to provide rail-

to-rail performance. This allows realistic simulation of open-

loop gain dependency on load resistance as well as maximum

output voltage versus output current. Two differential pairs are

used in the input stage of the model, simulating the rail-to-rail

input stage of the AD8591/AD8592/AD8594 amplifier.

The EOS voltage source establishes the input offset voltage and

is also used to simulate the common-mode rejection power

supply rejection, and input voltage noise characteristics for the

model. In addition, G2, R2 and CF are used to help set the

open-loop gain and gain-bandwidth product of the model.

A number of secondary characteristics are also accurately por-

trayed in the SPICE model. Flicker noise is accurately modeled

with the 1/f corner frequency set through the KF and AF terms

in the input stage transistors. C1 and C2 are used in the input

section to create secondary poles to achieve an accurate phase

margin characteristic for the model.

The AD8591/AD8592/AD8594 shutdown circuitry is included

in the model. Switches S1 through S7 deactivate the op amp

circuitry in shutdown mode. The logic threshold for the shut-

down circuitry is accurately modeled through the VSWITCH

model parameters near the end of the listing. The active supply

current versus supply voltage is also modeled through the volt-

age-controlled current source GSY.

Characteristics of this model are based on typical values for the

AD8591/AD8592/AD8594 amplifier at +27

°C. The model’s

characteristics are optimized specifically at +27

°C, and may lose

accuracy at different simulation temperatures.