ADA4930-1/ADA4930-2
Rev. A | Page 18 of 28
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
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
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
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)
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.
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.