8.4 Device Functional Modes

8.4.1 Split-Supply and Single-Supply Operation

The OPA855-Q1 can be configured with single-sided supplies or split-supplies as shown in Figure 10-1. Split-

supply operation using balanced supplies with the input common-mode set to ground eases 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, the thermal pad must be connected 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-Q1 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, the thermal pad must be connected to ground.

8.4.2 Power-Down Mode

The OPA855-Q1 features a power-down mode to reduce the quiescent current to conserve power. Figure 6-23

and Figure 6-24 show the transient response of the OPA855-Q1 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 and enable

threshold voltages are 0.65 V and 1.8 V, respectively. If the amplifier is configured with ±1.65 V supplies, then

the threshold voltages are at –1 V and 0.15 V. If the amplifier is configured with ±2.5 V supplies, then the

threshold voltages are at –1.85 V and –0.7 V.

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

state 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 state to the enabled state. The small difference in the switching thresholds

between the down sweep and the up sweep is caused by the hysteresis designed into the amplifier to increase

immunity to noise on the PD pin.

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

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

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

parallel load to the output of the amplifier. To protect the input stage of the amplifier, the OPA855-Q1 uses

internal, back-to-back protection diodes between the inverting and noninverting input pins as Figure 8-3 shows.

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-Q1

SBOSA57A – FEBRUARY 2021 – REVISED MARCH 2021

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