The OPAx591 is used to build a high precision instrumentation amplifier that is capable of operating with high

common-mode voltages. This instrumentation is an attractive option for high voltage, high-side current sensing.

A high-voltage, high-side current sense application is discussed here.

Use the following parameters for this design example:

• Common-mode voltage: 54V

• Current output range: 100nA to 1µA

• Typical uncalibrated error: < 0.1%

7.2.1.2 Detailed Design Procedure

The OPAx591 can be configured as a three operational amplifier (op amp) to achieve a very high input

common-mode voltage range. The instrumentation amplifier can be compartmentalized as a two stage amplifier

with an input or gain stage, and a difference amplifier stage. Figure 7-2 illustrates a generic three op amp

instrumentation amplifier.

+

–

VIN

VOUT

R

+

–

VIN+

RG

RF2

RF1

R

+

–

R

R

OPA

OPA

V+

V+

V+

OPA

VREF

Gain stage

Difference amplifier

Figure 7-2. Instrumentation Amplifier

To support the high common-mode voltage range requirement, the OPAx591 is powered with 85V, yielding a

81.5V input common-mode voltage. The OPAx591 has an input voltage range of (V−) − 0.1V to (V+) − 3.5V.

The output voltage swing also limits the common-mode voltage for a given differential voltage. The relationship

is given below in Equation 3. Note that 85V is the maximum operating power supply voltage of the OPAx591 and

the 1.5V accounts for the linear output voltage swing for 10kΩ as given in Electrical Characteristics.

VIN++ VIN+−VIN− RFRG <85V−1.5V

(3)

The shunt resistor is set to be as small as possible to minimize the voltage drop. Given practical amplifier error

sources, choose a shunt resistor that provides a voltage drop large enough to meet the accuracy requirements

of the application. The largest error source in current sense applications is the amplifier and resistor drift and

typically set the lower bound on the shunt resistor value. In most general purpose applications, a voltage drop of

no more than 100mV is acceptable.

The OPAx591 provides very low offset and offset drift to enable high accuracy measurements to be made even

at low current values. Another benefit of the OPAx591 is the low input bias current of 15pA, maximum. The low

input bias current enables high accuracy measurements down to the nanoampere range.

OPAx591 circuit is configured to provide a maximum output voltage of 20V for a full scale current of 1µA,

corresponding to a 100mV drop across the 10kΩ shunt resistor. The gain of the first stage is set to 20V/V and

the difference amplifier stage gain is set to 10V/V for a total gain of 200V/V. The lower bound of the current

measurement capability of the circuit is determined by the maximum error tolerance of the application such that

the percent error of the circuit increases as the voltage drop across the shunt resistor decreases. The transfer

function of the amplifier given by Equation 4.

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