ADA4941-1YRZ-R7 datasheet(20/24 Pages) AD

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ADA4941-1

Rev. C | Page 20 of 24

APPLICATIONS

OVERVIEW

The ADA4941-1 is an adjustable-gain, single-ended-to-differential

voltage amplifier, optimized for driving high resolution ADCs.

Single-ended-to-differential gain is controlled by one feedback

network, comprised of two external resistors: RF and RG.

USING THE REF PIN

The REF pin sets the output base line in the inverting path and

is used as a reference for the input signal. In most applications,

the REF pin is set to the input signal midswing level, which in

many cases is also midsupply. For bipolar signals and dual

power supplies, REF is generally set to ground. In single-supply

applications, setting REF to the input signal midswing level

provides optimal output dynamic range performance with

minimum differential offset. Note that the REF input only

affects the inverting signal path or VON.

Most applications require a differential output signal with the

same dc common-mode level on each output. It is possible for

the signal measured across VOP and VON to have a common-

mode voltage that is of the desired level but not common to

both outputs. This type of signal is generally avoided because

it does not allow for optimal use of the amplifier’s output

dynamic range.

Defining VIN as the voltage applied to the input pin, the

equations that govern the two signal paths are given in

Equation 21 and Equation 22.

VOP = VIN

(21)

VON = −VIN + 2 (REF)

(22)

When the REF voltage is set to the midswing level of the input

signal, the two output signals fall directly on top of each other

with minimal offset. Setting the REF voltage elsewhere results

in an offset between the two outputs.

The best use of the REF pin can be further illustrated by

considering a single-supply case with a 10 V power supply and

an input signal that varies between 2 V and 7 V. This is a case

where the midswing level of the input signal is not at midsupply

but is at 4.5 V. Setting the REF input at 4.5 V and neglecting

offsets, Equation 21 and Equation 22 are used to calculate the

results. When the input signal is at its midpoint of 4.5 V,

OUT+ is at 4.5 V, as is VON. This can be considered as a base

line state where the differential output voltage is 0. When the

input increases to 7 V, VOP tracks the input to 7 V, and VON

decreases to 2 V. This can be viewed as a positive peak signal

where the differential output voltage equals 5 V. When the input

signal decreases to 2 V, VOP again tracks to 2 V, and VON

increases to 7 V. This can be viewed as a negative peak signal

where the differential output voltage equals −5 V. The resulting

differential output voltage is 10 V p-p.

The previous discussion reveals how the single-ended-to-

differential gain of 2 is achieved.

INTERNAL FEEDBACK NETWORK POWER

DISSIPATION

While traditional op amps do not have on-chip feedback

elements, the ADA4941-1 contains two on-chip, 1 kΩ resistors

that comprise an internal feedback loop. The power dissipated

in these resistors must be included in the overall power dissipation

calculations for the device. Under certain circumstances, the

power dissipated in these resistors could be comparable to the

device’s quiescent dissipation. For example, on ±5 V supplies

with the REF pin tied to ground and OUT− at +4 VDC, each

1 kΩ resistor carries 4 mA and dissipates 16 mW for a total of

32 mW. This is comparable to the quiescent power and must

therefore be included in the overall device power dissipation

calculations. For ac signals, rms analysis is required.

DISABLE FEATURE

The ADA4941-1 includes a disable feature that can be asserted

to minimize power consumption in a device that is not needed

at a particular time. When asserted, the disable feature does not

place the device output in a high impedance or tristate condition.

The disable feature is active high. See the Specifications tables

for the high and low level voltage specifications.

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