ADA4941-1YRZ-R7 datasheet(15/24 Pages) AD

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ADA4941-1

Rev. C | Page 15 of 24

THEORY OF OPERATION

The ADA4941-1 is a low power, single-ended input, differential

output amplifier optimized for driving high resolution ADCs.

Figure 47 illustrates how the ADA4941-1 is typically connected.

The amplifier is composed of an uncommitted amplifier, A1,

driving a precision inverter, A2. The negative input of A1 is

brought out to Pin 1 (FB), allowing for user-programmable

gain. The inverting op amp, A2, provides accurate inversion of

the output of A1, VOP, producing the output signal VON.

Ω

RF || RG

500

Ω

REF

VREF

OUT+

–

VOP

OUT–

–

VON

VIN

Figure 47. Basic Connections (Power Supplies Not Shown)

The voltage applied to the REF pin appears as the output

common-mode voltage. Note that the voltage applied to the

REF pin does not affect the voltage at the OUT+ pin. Because of

this, a differential offset can exist between the outputs, while the

desired output common-mode voltage is present. For example,

when VOP = 3.5 V and VON = 1.5 V, the output common-

mode voltage is equal to 2.5 V, just as it is when both outputs

are at 2.5 V. In the first case, the differential voltage (or offset) is

2.0 V, and in the latter case, the differential voltage is 0 V. When

calculating output voltages, both differential and common-mode

voltages must be considered at the same time to avoid undesired

differential offsets.

BASIC OPERATION

In Figure 47, RG and RF form the external gain-setting network.

VG and VREF are externally applied voltages. VO, cm is defined

as the output common-mode voltage and VO, dm is defined as

the differential-mode output voltage. The following equations

can be derived from Figure 47:













−













VIN

VOP

(1)

)

(

VREF

VIN

VON

























−

(2)

)

(

)

(

VREF

VIN

VON

VOP

−













−













−

(3)

VREF

VON

VOP













(4)

When RF = 0 and RG is removed, Equation 3 simplifies to the

following:

VO, dm = 2(VIN) − 2(VREF)

(5)

Ω

4.99k

Ω

825

Ω

500

Ω

REF

+5V

–5V

VS+

VS–

OUT+

–

VOP

OUT–

–

VON

VIN

Figure 48. Dual Supply, G = 2.4, Single-Ended-to-Differential Amplifier

Figure 48 shows an example of a dual-supply connection. In this

example, VG and VREF are set to 0 V, and the external RF and

RG network provides a noninverting gain of 1.2 in A1. This

example takes full advantage of the rail-to-rail output stage.

The gain equation is

VOP − VON = 2.4(VIN)

(6)

The in-series, 825 Ω resistor combined with Pin 8 compensates

for the voltage error generated by the input offset current of A1.

The linear output range of both A1 and A2 extends to within

200 mV of each supply rail, which allows a peak-to-peak

differential output voltage of 19.2 V on ±5 V supplies.

Ω

500

Ω

REF

+5V

VS+

VS–

OUT+

–

VOP

OUT–

–

VON

+2.5V

VIN

Figure 49. Single +5V Supply, G=2 Single-Ended-to-Differential Amplifier

Figure 49 shows a single 5 V supply connection with A1 used as

a unity gain follower. The 2.5 V at the REF pin sets the output

common-mode voltage to 2.5 V. The transfer function is then

VOP − VON = 2(VIN) − 5 V

(7)

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