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# EDNE JAN 2017

Analog Tips
A termination resistor in parallel with the input
can be used if required.
The ADA4940-1internal common-mode
feedback loop forces common-mode output
voltage to equal the voltage applied to the VOCM
input and offers an excellent output balance.
The differential output voltage depends on VOCM
when two feedback factors β1 and β2 are not
equal and any imbalance in output amplitude
or phase produces an undesirable commonmode
component in the output and causes a
redundant noise and offset in the differential
output. Therefore, it’s imperative that the
combination of input source impedance and
R1 (R3) should be 1 kΩ in this case (that is, β1
= β2) to avoid the mismatch in the commonmode
voltage of each output signal and prevent
the increase in common-mode noise coming
from the ADA4940-1.
As signals travel through the traces of a
printed-circuit board (PCB) and long cables,
system noise accumulates in the signals and a
differential input ADC rejects any signal noise
that appears as a common-mode voltage.
The expected signal-to-noise ratio (SNR) of this
18-bit, 1 MSPS data acquisition system can be
calculated theoretically by taking the root sum
square (RSS) of each noise source – ADA4940-
1, ADR435 and AD7982.
The ADA4940-1offers low noise performance
of typically 3.9 nV/√Hz at 100 kHz as shown in
Figure 2.
Figure 2 ADA4940 input voltage noise spectral
density vs. frequency.
It is important to calculate the noise gain of
the differential amplifier in order to find its
equivalent output noise contribution. The noise
gain of the differential amplifier is:
where;
and
are two feedback factors.
The following differential amplifier noise
sources should be taken into account:
Since the ADA4940-1 input voltage noise is 3.9
nV/√Hz, its differential output noise would be
7.8 nV/√Hz. The ADA4940-1’s common-mode
input voltage noise (eOCM) is 83 nV/√Hz from the
data sheet, so its output noise would be;
– eOCM × (β1 – β2) × NG = 0
Noise from the R1, R2, R3, and R4 resistors
can be calculated based on the Johnson-
Nyquist noise equation for a given bandwidth.
eRn= √(4kB TR)
where kB is the Boltzmann constant, (1.38065
× 10-23 J/K), T is the resistor’s absolute
temperature in Kelvin, and R is the resistor
value in ohms (Ω).
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EDNE JAN 2017

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