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8.3.3 Output Clamping

The INA851 features a unique output clamping function that protects against damage that results from

inadvertent overdriving of the ADC. 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, 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 8-5. This behavior becomes noticeable in the form of output distortion when the output voltage

exceeds approximately 100 mV beyond the clamp voltage. The output is typically unable to swing greater

than approximately 600 mV beyond the VCLAMP+ or VCLAMP– voltage (depending on the output current), helping

prevent or reduce damage to the ADC from overvoltage conditions.

Output Driver

VS+

OUT+

OUT

VCLAMP+

VCLAMP

–

FDA_IN

FDA_IN+

FDA Input

–

Predriver

–

Figure 8-5. 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 after the configured 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 mV before the clamps turn on strongly. When used in conjunction

with several 10s 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), this feature helps severely

diminish any potential damage to the ADC that might otherwise result.

The VCLAMP+ pin features additional overvoltage protection as a fail-safe against power-up sequencing issues

between the INA851 and a downstream device. For example, a condition of VS+ + 10 V on this pin does not

cause damage; therefore, the ADC supply can safely turn on while the INA851 supply is still off or just beginning

to turn on. In this case, the current draw through the VCLAMP+ pin is limited to a safe value.

8.4 Device Functional Modes

The INA851 has a single functional mode and operates when the power-supply voltage is greater than 8 V

(±4 V). The maximum power-supply voltage for the INA851 is 36 V (±18 V).

INA851

SBOS999 – MARCH 2022

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