CS5371A CS5372A
DS748F3 17
4.3 Anti-alias Filter
The modulator inputs are required to be band-
width limited to ensure modulator loop stability
and prevent high-frequency signals from alias-
ing into the measurement bandwidth. The use
of simple, single-pole, differential, low-pass
RC filters across the INR± and INF± inputs en-
sures high-frequency signals are rejected be-
fore they can alias into the measurement
bandwidth.
The CS3301A / CS3302A differential amplifi-
ers are designed with separate rough and fine
analog outputs (OUTR±, OUTF±) that match
the modulator rough and fine inputs (INR±,
INF±). External anti-alias series resistors and
external differential capacitors are required to
create the anti-alias RC filters.
The approximate -3 dB corner of the input anti-
alias filter is nominally set to the internal ana-
log sampling rate divided by 64, which itself is
a division by 4 of the MCLK rate.
Figure 9 on page 13 illustrates the CS5371A and
CS5372A modulator analog connections with
input anti-alias filter components. Filter com-
ponents on the rough and fine pins should be
identical values for optimum performance, with
the capacitor values a minimum of 0.02 μF.
The rough input can use either X7R- or C0G-
type capacitors, while the fine input requires
C0G-type capacitors for optimal linearity. Us-
ing X7R-type capacitors on the fine analog in-
puts will significantly degrade total harmonic
distortion performance.
4.4 Analog Differential Signals
Differential analog signals into the CS5371A
and CS5372A consist of two halves with equal
but opposite magnitude varying about a com-
mon mode voltage. A full-scale, 5 V
P-P
, differ-
ential signal centered on a -0.15 V common
mode voltage will have:
SIG+ = -0.15 V + 1.25 V = +1.1 V
SIG- = -0.15 V - 1.25 V = -1.4 V
SIG+ is +2.5 V relative to SIG-
For the opposite case:
SIG+ = -0.15 V - 1.25 V = -1.4 V
SIG- = -0.15 V + 1.25 V = +1.1 V
SIG+ is -2.5 V relative to SIG-
So the total swing for SIG+ relative to SIG- is
(+2.5 V) – (-2.5 V) = 5 V
p-p
differential. A simi-
lar calculation can be done for SIG- relative to
SIG+.
It’s important to note that a 5 V
p-p
differential
signal centered on a -0.15 V common mode
voltage never exceeds +1.1 V with respect to
ground and never drops below -1.4 V with re-
spect to ground on either half. By definition,
differential voltages are measured with re-
spect to the opposite half, not relative to
ground. A voltmeter differentially measuring
between SIG+ and SIG- in the above example
would correctly read 1.767 V
rms
, or 5 V
p-p
.
• MCLK Frequency = 2.048 MHz
• Sampling Frequency = MCLK / 4 = 512 kHz
• -3 dB Filter Corner = Sampling Freq / 64 = 8 kHz
• RC filter = 1 / [ 2π x(2xR
series
)xC
diff
] ~ 8 kHz
