INA821 datasheet(28/52 Pages) TI

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8.3.5 Output Clamping

The INA851 features a unique output clamping function that protects the downstream device against damage

that results from inadvertent over-driving. Usually the downstream device is an ADC that typically operates at a

lower supply voltage than the INA851.

To implement this function, use the VCLAMP+ and VCLAMP– pins to limit the supply voltage range of the

differential output drive amplifier. For typical operation, use a low-impedance connection from the pins to the

power supplies of the ADC. For proper operation of the clamp circuitry, set the VS+ or VS– supply voltages at

least 1.5 V beyond the respective VCLAMP+ and VCLAMP– clamping voltages. In addition, for the output driver

to function correctly, the VCLAMP+ and VCLAMP– voltages must be at least 3 V apart. If the output clamping

functionality is not desired, short the VCLAMP+ and VCLAMP– pins to the amplifier VS+ and VS– supply pins.

The output driver operates up to the VCLAMP+ or VCLAMP– limits without experiencing distortion. When the

clamping function is in use the output voltage is clamped at approximately 600 mV beyond the clamp voltage.),

so that the device output spans the full input range of the ADC. However, if the predriver output swings beyond

the VCLAMP+ or VCLAMP– voltage, the output driver begins to run out of headroom as a result of the clamped

supply voltage, shown in Figure 8-12.

The output is unable to swing greater than approximately 500 mV beyond the VCLAMP+ or VCLAMP– voltage (at

zero load), helping prevent or reduce damage to the ADC from overvoltage conditions.

The linear operation becomes distorted when the driver load becomes higher than 20 mA, see Figure 8-13.

Output Driver

VS+

OUT+

OUT

VCLAMP+

VCLAMP

–

FDA_IN

FDA_IN+

FDA Input

–

Predriver

–

Figure 8-12. Simplified Schematic of Output Driver Clamping Structure

Note

Be aware that instead of providing an immediate hard-stop voltage limit, the output driver is not

clamped until VCLAMP+ or VCLAMP– voltage has been exceeded, and the driver starts to run out of

headroom. This configuration prevents distortion of the amplifier output when operating near the

VCLAMP+ and VCLAMP– rails. However, the output voltage exceeds VCLAMP+ and VCLAMP– by several

hundreds of millivolts before the clamps turn on strongly. When used in conjunction with several tens

of ohms of resistance between the amplifier output and ADC input pins (commonly implemented as

part of a low-pass filter for proper ADC acquisition), output clamping helps severely diminish any

potential damage to the ADC that can otherwise result.

INA851

SBOS999A – MARCH 2022 – REVISED OCTOBER 2022

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