ADA4930-1/ADA4930-2

Data Sheet

Rev. D | Page 18 of 25

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 Terms

Differential Input

vnIN

vnIN

GN

vnOD1 = GN(vnIN)

Inverting Input

inIN+

inIN+ × (RF2)

1

vnOD2 = (inIN+)(RF2)

Noninverting Input

inIN−

inIN− × (RF1)

1

vnOD3 = (inIN−)(RF1)

VOCM Input

vnCM

vnCM

0

vnOD4 = 0

Gain Resistor RG1

vnRG1

(4kTRG1)1/2

RF1/RG1

vnOD5 = (RF1/RG1)(4kTRG1)1/2

Gain Resistor RG2

vnRG2

(4kTRG2)1/2

RF2/RG2

vnOD6 = (RF2/RG2)(4kTRG2)1/2

Feedback Resistor RF1

vnRF1

(4kTRF1)1/2

1

vnOD7 = (4kTRF1)1/2

Feedback Resistor RF2

vnRF2

(4kTRF2)1/2

1

vnOD8 = (4kTRF2)1/2

Table 12. Differential Input, DC-Coupled, VS = 5 V

Nominal Gain (dB)

RF1, RF2 (Ω)

RG1, RG2 (Ω)

RIN, dm (Ω)

Differential Output Noise Density (nV/√Hz)

0

301

301

602

4.9

6

301

150

300

6.2

10

301

95.3

190.6

7.8

14

301

60.4

120.4

10.1

Table 13. Single-Ended Ground-Referenced Input, DC-Coupled, RS = 50 Ω, VS = 5 V

Nominal Gain (dB)

RF1, RF2 (Ω)

RG1 (Ω)

RT (Ω)

RIN, cm (Ω)

RG2 (Ω)1

Differential Output Noise Density (nV/√Hz)

0

301

142

64.2

190.67

170

5.9

6

301

63.4

84.5

95.06

95

7.8

10

301

33.2

1 k

53.54

69.3

9.3

14

301

10.2

1.15 k

17.5

57.7

10.4

1 RG2 = RG1 + (RS||RT).

Table 11 summarizes the input noise sources, the multiplication

factors, and the output-referred noise density terms.

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 0.9 V,

a differential offset voltage is present at the output of

(0.9 V)(0.67) = 0.6 V. The differential output noise contribution

is (5 nV/√Hz)(0.67) = 3.35 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 9 mV due

to a 0.9 V VOCM input, negligible VOCM noise contribution, and

no significant degradation in output balance error.

INPUT COMMON-MODE VOLTAGE RANGE

The input common-mode range at the summing nodes of the

ADA4930-1/ADA4930-2 is specified as 0.3 V to 1.5 V at VS = 3.3 V.

To avoid nonlinearities, the voltage swing at the +IN and −IN

terminals must be confined to these ranges.