Uses of the OpAmp Macromodel

You can use the OpAmp macromodel by doing the following:

• Defining the common specification sheet values, available in most semiconductor product catalogs
• Placing the OpAmp symbol in your schematic
• Defining the CDF parameter values and by using default values for parameters that you do not set

The OpAmp macromodel might not work as a comprehensive model. Its limitations are described here:

• Macromodel accuracy
  The OpAmp macromodel is less accurate and comprehensive than the transistor level representation, and even less accurate than the circuit measurements.
• Model scope
  Specifications not defined as macromodel parameters are not modeled, including common-mode input range, noise, transient power dissipation, and output impedance.
• Temperature
  Temperature effects are not implicitly included, but can be explicitly defined as an algebraic or table function. For example:

  offset voltage = 1m + 0.01 m * (tempdc - 25)

  OR

  offset voltage = TABLE(tempdc,-75,0,25,1m,125,2m)

• Parameter independence
  To a first order of accuracy, you can set the macromodel parameters independently, ignoring interactions.
• Small input bias currents
  When using small input bias currents (~1E-12), decrease the simulation control variable GMIN (minimum conductance) to ~1E-15 (default is 1.0E-12). This helps to prevent relatively large leakage currents and corresponding accuracy loss.
• Unrealistic conditions
  The macromodel is particularly good for modeling the OpAmp components. Avoid the following unrealistic parameter values or extreme conditions:
  • Offset voltages less than 10 mvolts vary due to numerical errors
  • The following quantities affect the accuracy of the slew rate of the macromodel in small but measurable ways:
    • Gain-bandwidth product (GBW)
    • Large-signal saturation delay (TSD)
    • Second pole frequency (FF2)
    
    This effect can be significant for the following examples of extreme cases:
    • TSD<<(1/GBW)
    • FF2<<GBW
    
    In these situations, measure the simulated macromodel response and adjust the parameters accordingly.
• Additional parameter requirements
  • The slew rates must conform to the inequality 0.1 ≤ srn/srp ≤ 10
  • The output DC and AC resistances must conform to the inequality rod > roa > 0

Setup of the OpAmp Macromodel into a Design

The following figure shows an OpAmp macromodel symbol:

For proper operation of the macromodel, you need to connect the following six I/O pins:

IN+	Positive differential input pin
IN-	Negative differential input pin
OUT	OpAmp output pin
VCC	Positive power supply pin
VEE	Negative power supply pin
REF	Reference voltage (typically ground)

OpAmp Macromodel Parameters

There are 23 parameters you can use to describe the DC, AC, and transient behavior of the OpAmp in the macromodel.

Property	Typical Value	Description
ccc	30 p farads	Internal compensation capacitor
ibs	100 n amps	Input bias current
ios	10 n amps	Input offset current
vos	1 m+.01 m* (tempdc-25) volts	Input offset voltage
srp	10 me volts/second	Positive slew rate
srn	10 me volts/second	Negative slew rate
ftp	0.1	Positive feedthrough
gol	100 k volts/volt	Open loop gain
cmr	90 db	Common mode rejection ratio
gbw	10 MHz	Gain-bandwidth product
ff2	20 MHz	Second pole frequency
tsd	500 n seconds	Overdrive storage delay
roa	20 ohms	AC output resistance
rod	30 ohms	DC output resistance
vsn	-14 volts	Negative output swing limit
vsp	14 volts	Positive output swing limit
ion	50 m amps	Negative output current limit
iop	50 m amps	Positive output current limit
prn	90 db	Negative power supply rejection ratio
prp	90 db	Positive power supply rejection ratio
iee	5 m amps	Negative power supply current
icc	5 m amps	Positive power supply current
vnm	15 volts	Nominal power supply

You define these parameters in the cell CDF description.

The property value can be a

• Number
• Variable
• Valid expression (with variables)

For example, to define the offset voltage as a simple linear function of temperature, enter the following expression as the value of the offset voltage property:

vos = 1m + 0.01m * (TEMPDC - 25)

Procurement of OpAmp Macromodel

You can obtain model parameters in several ways, depending on your application.

• IC vendor catalog
  Obtain most of the parameters from the specific amplifier catalog data sheet
• Schematics
  Simulate and measure the performance parameters you want using the OpAmp transistor schematic and associated SPICE device models
• Component
  Make the measurements in the lab using the OpAmp component

You can use the default values or set some reasonable values for the remaining parameters. For example, you might set parameters to some typical values (or use the defaults) when exploring your system design space before selecting a particular amplifier, then modify the values later.

OpAmp System Size

A primary reason for using macromodels in large systems is for faster simulations. These simulations are faster when compared to transistor-level simulations. These simulations are about 10 to 50 times faster for complex OpAmps containing between 30 and 100 devices. Storage requirements are also reduced using macromodels.

The fast simulation time and reduced storage requirement of smaller amplifiers (fewer than 10 devices) might not compensate for the loss of accuracy from using macromodels. However, macromodels are still useful for exploratory tuning and for protecting libraries.

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