ADA4927-2YCPZ-R2 datasheet(18/24 Pages) AD

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ADA4927-1/ADA4927-2

Rev. 0 | Page 18 of 24

Table 12. Differential Input, DC-Coupled

Nominal Gain (dB)

RF (Ω)

RG (Ω)

RIN, dm (Ω)

Differential Output Noise Density (nV/√Hz)

301

602

8.0

442

44.2

88.4

21.8

604

30.1

60.2

37.9

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

Nominal Gain (dB)

RF (Ω)

RG1 (Ω)

RT (Ω)

RIN, cm (Ω)

RG2 (Ω)1

Differential Output Noise Density (nV/√Hz)

309

301

56.2

401

328

8.1

511

39.2

158

73.2

77.2

18.6

806

649

54.2

74.4

29.1

1 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 +IN and −IN 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 zero 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 several common gain settings, associated

resistor values, input impedance, and output noise density for

both balanced and unbalanced input configurations.

IMPACT OF MISMATCHES IN THE FEEDBACK

NETWORKS

As previously mentioned, even if the external feedback networks

(RF/RG) are mismatched, the internal common-mode feedback

loop still forces the outputs to remain balanced. The amplitudes

of the signals at each output remain equal and 180° out of phase.

The input-to-output differential mode gain varies proportionately

to the feedback mismatch, but the output balance is unaffected.

The gain from the VOCM pin to VO, dm is equal to

2(β1 − β2)/(β1 + β2)

When β1 = β2, this term goes to zero and there is no differential

output voltage due to the voltage on the VOCM input (including

noise). The extreme case occurs when one loop is open and the

other has 100% feedback; in this case, the gain from VOCM input

to VO,dm is either +2 or −2, depending on which loop is closed.

The feedback loops are nominally matched to within 1% in

most applications, and the output noise and offsets due to the

VOCM input are negligible. If the loops are intentionally mismatched

by a large amount, it is necessary to include the gain term from

VOCM to VO, dm and account for the extra noise. For example, if

β1 = 0.5 and β2 = 0.25, the gain from VOCM to VO, dm is 0.67. If the

VOCM pin is set to 2.5 V, a differential offset voltage is present at the

output of (2.5 V)(0.67) = 1.67 V. The differential output noise

contribution is (15 nV/√Hz)(0.67) = 10 nV/√Hz. Both of these

results are undesirable in most applications; therefore, it is best

to use nominally matched feedback factors.

Mismatched feedback networks also result in a degradation of

the ability of the circuit to reject input common-mode signals,

much the same as for a four-resistor difference amplifier made

from a conventional op amp.

As a practical summarization of the previous issues, resistors of

1% tolerance produce a worst-case input CMRR of approximately

40 dB, a worst-case differential-mode output offset of 25 mV

due to a 2.5 V VOCM input, negligible VOCM noise contribution,

and no significant degradation in output balance error.

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 48, the input impedance (RIN, dm) between the inputs

(+DIN and −DIN) is simply RIN, dm = RG + RG = 2 × RG.

+VS

–VS

ADA4927

+IN

–IN

+DIN

–DIN

VOCM

VOUT, dm

Figure 48. The ADA4927 Configured for Balanced (Differential) Inputs

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