8.4 Device Functional Modes

8.4.1 Split-Supply and Single-Supply Operation

The OPA855 can be configured with single-sided supplies or split supplies; see also Figure 9-11 . Split-supply

operation using balanced supplies with the input common-mode set to ground can help ease lab testing

(because most signal generators, network analyzers, spectrum analyzers, and other lab equipment typically

reference inputs and outputs to ground). In split-supply operation, connect the thermal pad to the negative

supply.

Newer systems use a single power supply to improve efficiency and reduce the cost of the extra power

supply. The OPA855 can be used with a single positive supply (negative supply at ground) with no change in

performance if the input common-mode and output swing are biased within the linear operation of the device. In

single-supply operation, level shift the dc input and output reference voltages by half the difference between the

power supply rails. This configuration maintains the input common-mode and output load reference at midsupply.

To eliminate gain errors, the source driving the reference input common-mode voltage must have low output

impedance across the frequency range of interest. In this case, connect the thermal pad to ground.

8.4.2 Power-Down Mode

The OPA855 features a power-down mode to reduce the quiescent current to conserve power. Figure 6-23 and

Figure 6-24 show the OPA855 transient response as the PD pin toggles between the disabled and enabled

states.

The PD disable and enable threshold voltages are with reference to the negative supply. If the amplifier is

configured with the positive supply at 3.3 V and the negative supply at ground, then the disable threshold voltage

is 0.65 V and the enable threshold voltage is 1.8 V. If the amplifier is configured with ±1.65-V supplies, then the

disable threshold voltage is –1 V and the enable threshold voltage is 0.15 V. If the amplifier is configured with

±2.5-V supplies, then the disable threshold voltage is –1.85 V and the enable threshold voltage is –0.7 V.

Figure 8-8 shows the switching behavior of a typical amplifier as the PD pin is swept down from the enabled to

the disabled state. Similarly, Figure 8-9 shows the switching behavior of a typical amplifier as the PD pin is swept

up from the disabled to the enabled state. The small difference in the switching thresholds between the down

sweep and up sweep is due to the hysteresis designed into the amplifier to increase noise immunity on PD.

Power Down Voltage (V)

-2

-1.5

-1

-0.5

0

0.5

1

1.5

2

0

2.5

5

7.5

10

12.5

15

17.5

20

D600

TA = 40qC

TA = +25qC

TA = +125qC

Figure 8-8. Switching Threshold

(PD Pin Swept From High to Low)

Power Down Voltage (V)

-2

-1.5

-1

-0.5

0

0.5

1

1.5

2

0

2.5

5

7.5

10

12.5

15

17.5

20

D601

TA = 40qC

TA = +25qC

TA = +125qC

Figure 8-9. Switching Threshold ( PD Pin Swept

From Low to High)

Connecting the PD pin low disables the amplifier and places the output in a high-impedance state. When the

amplifier is configured as a noninverting amplifier, the feedback (RF) and gain (RG) resistor network form a

parallel load to the output of the amplifier. To protect the input stage of the amplifier, the OPA855 uses internal,

back-to-back protection diodes between the inverting and noninverting input pins; see also Figure 8-3. In the

power-down state, if the differential voltage between the input pins of the amplifier exceeds a diode voltage drop,

an additional low-impedance path is created between the noninverting input pin and the output pin.

OPA855

SBOS622D – JULY 2018 – REVISED MAY 2025

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