ADA4939-2YCPZ-R2 datasheet(18/24 Pages) AD

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ADA4939-1/ADA4939-2

Data Sheet

Rev. A | Page 18 of 24

Table 11. Output Noise Voltage Density Calculations for Matched Feedback Networks

Input Noise Contribution

Input Noise Term

Input Noise

Voltage Density

Output

Multiplication Factor

Differential Output Noise

Voltage Density Term

Differential Input

vnIN

vnO1 = GN(vnIN)

Inverting Input

inIN

inIN × (RF2)

vnO2 = (inIN)(RF2)

Noninverting Input

inIN

inIN × (RF1)

vnO3 = (inIN)(RF1)

VOCM Input

vnCM

vnO4 = 0

Gain Resistor RG1

vnRG1

(4kTRG1)1/2

RF1/RG1

vnO5 = (RF1/RG1)(4kTRG1)1/2

Gain Resistor RG2

vnRG2

(4kTRG2)1/2

RF2/RG2

vnO6 = (RF2/RG2)(4kTRG2)1/2

Feedback Resistor RF1

vnRF1

(4kTRF1)1/2

vnO7 = (4kTRF1)1/2

Feedback Resistor RF2

vnRF2

(4kTRF2)1/2

vnO8 = (4kTRF2)1/2

Table 12. Differential Input, DC-Coupled

Nominal Gain (dB)

RF (Ω)

RG (Ω)

RIN, dm (Ω)

Differential Output Noise Density (nV/√Hz)

402

200

400

9.7

402

127

254

12.4

402

80.6

161

16.6

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)

402

200

60.4

301

228

9.1

402

127

66.5

205

155

11.1

402

80.6

76.8

138

111

13.5

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/VOCMx 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/VOCMx 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 (7.5 nV/√Hz)(0.67) = 5 nV/√Hz. Both of these

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

to use nominally matched feedback factors.

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