ADA4950-2YCPZ-R7 datasheet(20/26 Pages) AD

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ADA4950-1/ADA4950-2

Data Sheet

Rev. B | Page 20 of 26

Table 12. Differential Input, DC-Coupled

Nominal Linear Gain

RF (Ω)

RG (Ω)

RIN, dm (Ω)

Differential Output Noise Density (nV/√Hz)

500

1000

9.25

500

250

500

12.9

500

250||500

333

16.6

Table 13. Single-Ended, Ground-Referenced Input, DC-Coupled, RS = 50 Ω

Nominal Linear Gain

RF (Ω)

RG1 (Ω)

RT (Ω) (Std 1%) RIN, se (Ω)

RG2 (Ω)1

Differential Output Noise Density (nV/√Hz)

500

53.6

667

526

9.07

500

250

57.6

375

277

12.2

500

250||500 61.9

267

194

15.0

RG2 = RG1 + (RS||RT).

Similar to the case of a conventional op amp, the output noise

voltage densities can be estimated by multiplying the input-

referred terms at +INx and −INx by the appropriate output

factor, where:

(

)

is the circuit noise gain.

and

are the feedback factors.

When the feedback factors are matched, RF1/RG1 = RF2/RG2,

β1 = β2 = β, and the noise gain becomes

Note that the output noise from VOCM goes to 0 in this case. The

total differential output noise density, vnOD, is the root-sum-

square of the individual output noise terms.

∑

nOi

nOD

Table 12 and Table 13 list the three available gain settings,

associated resistor values, input impedance, and output noise

density for both balanced and unbalanced input configurations.

CALCULATING THE INPUT IMPEDANCE FOR AN

APPLICATION CIRCUIT

The effective input impedance of a circuit depends on whether

the amplifier is being driven by a single-ended or differential

signal source. For balanced differential input signals, as shown

in Figure 54, the input impedance (RIN,dm) is

RIN, dm = (RG + RG) = 2 × RG

The value of RG depends on the selected gain.

+VS

–VS

+IN

–IN

VOCM

VOUT, dm

VIN, dm

ADA4950-x

Figure 54. ADA4950-x Configured for Balanced (Differential) Inputs

For an unbalanced, single-ended input signal (see Figure 55),

the input impedance is

(

) 













−

ADA4950-x

RL VOUT, dm

+VS

–VS

VOCM

RIN, se

Figure 55. ADA4950-x with Unbalanced (Single-Ended) Input

The input impedance of the circuit is effectively higher than it

is for a conventional op amp connected as an inverter because a

fraction of the differential output voltage appears at the inputs

as a common-mode signal, partially bootstrapping the voltage

across the input resistor, RG. The common-mode voltage at the

amplifier input terminals can be easily determined by noting

that the voltage at the inverting input is equal to the noninverting

output voltage divided down by the voltage divider that is formed

by RF and RG in the lower loop. This voltage is present at both

input terminals due to negative voltage feedback and is in phase

with the input signal, thus reducing the effective voltage across

RG in the upper loop and partially bootstrapping RG.

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