INA322EASLASH2K5 datasheet(12/22 Pages) TI1

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INA322

SBOS174B

OFFSET VOLTAGE ERROR CALCULATION

The offset voltage (VOS) of the INA322EA has a specified

maximum of 10mV with a +5V power supply and the

common-mode voltage at VS/2. Additional specifications

for power-supply rejection and common-mode rejection are

provided to allow the user to easily calculate worst-case

expected offset under the conditions of a given application.

Power Supply Rejection Ratio (PSRR) is specified in

µV/V.

For the INA322, worst case PSRR is 250

µV/V, which

means for each volt of change in power supply, the offset

may shift up to 250

µV. Common-Mode Rejection Ratio

(CMRR) is specified in dB, which can be converted to

µV/V using the following equation:

CMRR (in

µV/V) = 10[(CMRR in dB)/–20] • 106

For the INA322, the worst case CMRR over the specified

common-mode range is 60dB (at G = 25) or about

1mV/V This means that for every volt of change in com-

mon-mode, the offset will shift less than 1mV.

These numbers can be used to calculate excursions from the

specified offset voltage under different application condi-

tions. For example, an application might configure the

amplifier with a 3.3V supply with 1V common-mode. This

configuration varies from the specified configuration, repre-

senting a 1.7V variation in power supply (5V in the offset

specification versus 3.3V in the application) and a 0.65V

variation in common-mode voltage from the specified

VS/2.

Calculation of the worst-case expected offset would be as

follows:

Adjusted VOS = Maximum specified VOS +

(power-supply variation) • PSRR +

(common-mode variation) • CMRR

VOS = 10mV + (1.7V • 0.250mV/V) + (0.65V • 1mV/V)

±11.075mV

However, the typical value will be closer to 2.2mV (calcu-

lated using the typical values).

FEEDBACK CAPACITOR IMPROVES RESPONSE

For optimum settling time and stability with high-imped-

ance feedback networks, it may be necessary to add a

feedback capacitor across the feedback resistor, RF, as shown

in Figure 8. This capacitor compensates for the zero created

by the feedback network impedance and the INA322’s RG-

pin input capacitance (and any parasitic layout capacitance).

The effect becomes more significant with higher impedance

networks. Also, RX and CL can be added to reduce high-

frequency noise.

FIGURE 8. Feedback Capacitor Improves Dynamic Perfor-

mance.

It is suggested that a variable capacitor be used for the

feedback capacitor since input capacitance may vary be-

tween instrumentation amplifiers, and layout capacitance is

difficult to determine. For the circuit shown in Figure 8, the

value of the variable feedback capacitor should be chosen by

the following equation:

RIN • CIN = RF • CF

Where CIN is equal to the INA322’s RG-pin input capaci-

tance (typically 3pF) plus the layout capacitance. The ca-

pacitor can be varied until optimum performance is ob-

tained.

INA322

V+

OUT

IN • CIN = RF • CF

Where C

IN is equal to the INA322’s input capacitance

(approximately 3pF) plus any parastic layout capacitance.

Shutdown

IN–

V–

IN+

REF

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