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TERMINATING UNUSED AMPLIFIERS

Terminating unused amplifiers in a multiamplifier package is

an important step in ensuring proper operation of the functional

amplifier. Unterminated amplifiers can oscillate and draw

excessive power. The recommended procedure for terminating

unused amplifiers is to connect any unused amplifiers in a

unity-gain configuration and to connect the noninverting input

to midsupply voltage. With symmetrical bipolar power supplies,

this means connecting the noninverting input to ground, as

shown in Figure 59.

–VS

+VS

ADA4891

Figure 59. Terminating Unused Amplifier with

Symmetrical Bipolar Power Supplies

In single power supply applications, a synthetic midsupply

source must be created. This can be accomplished with a simple

resistive voltage divider. Figure 60 shows the proper connection

for terminating an unused amplifier in a single-supply

configuration.

2.5kΩ

+VS

ADA4891

Figure 60. Terminating Unused Amplifier with Single Power Supply

DISABLE FEATURE (ADA4891-3 ONLY)

The ADA4891-3 includes a power-down feature that can be

used to save power when an amplifier is not in use. When an

amplifier is powered down, its output goes to a high impedance

state. The output impedance decreases as frequency increases;

this effect can be observed in Figure 34. With the power-down

function, a forward isolation of −40 dB can be achieved at

50 MHz. Figure 46 shows the forward isolation vs. frequency

data. The power-down feature is asserted by pulling the PD1,

PD2, or PD3 pin low.

Table 7 summarizes the operation of the power-down feature.

Table 7. Disable Function

Power-Down Pin Connection (PDx)

Amplifier Status

>VTH or floating

Enabled

<VTH

Disabled

SINGLE-SUPPLY OPERATION

The ADA4891 can also be operated from a single power supply.

Figure 61 shows the ADA4891-3 configured as a single 5 V

supply video driver.

•

The input signal is ac-coupled into the amplifier via

Capacitor C1.

•

Resistor R2 and Resistor R4 establish the input midsupply

reference for the amplifier.

•

Capacitor C5 prevents constant current from being drawn

through the gain set resistor (RG) and enables the ADA4891-3

at dc to provide unity gain to the input midsupply voltage,

thereby establishing the output voltage at midsupply.

•

Capacitor C6 is the output coupling capacitor.

The large-signal frequency response obtained with single-

supply operation is identical to the bipolar supply operation

(Figure 18 shows the large-signal frequency response).

Four pairs of low frequency poles are formed by R2/2 and C2,

R3 and C1, RG and C5, and RL and C6. With this configuration,

the −3 dB cutoff frequency at low frequency is 12 Hz. The

values of C1, C2, C5, and C6 can be adjusted to change the low

frequency −3 dB cutoff point to suit individual design needs.

For more information about single-supply operation of op amps,

see the Analog Dialogue article “Avoiding Op Amp Instability

Problems in Single-Supply Applications” (Volume 35, Number 2)

at www.analog.com.

1µF

50kΩ

100kΩ

22µF

50Ω

22µF

150Ω

453Ω

22µF

ADA4891-3

+5V

VOUT

VIN

–VS

10µF

0.01µF

+5V

Figure 61. Single-Supply Video Driver Schematic

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