Encoder Group Delay; Design Considerations; Analog Inputs - Analog Devices AD73360L Manual

AD73360L
Figure 23 shows a comparison of SNR results achieved by vary-
ing either the Decimation Rate Setting or the DMCLK Rate
Settings.
81
80
79
78
77
76
75
74
73
72
71
8 16 24
SAMPLING FREQUENCY – kHz
Figure 23. Comparison of DMCLK and Decimation Rate
Settings

Encoder Group Delay

The AD73360L implementation offers a very low level of group
delay, which is given by the following relationship:
Group Delay (Decimator) = Order × ((M – 1)/2) × Tdec
where:
Order is the order of the decimator (= 3),
M is the decimation factor (= 32), and
Tdec is the decimation sample interval (= 1/2.048e6).
=> Group Delay (Decimator) = 3 × (32 – 1)/2 × (1/2.048e6)
= 22.7 µs
If final filtering is implemented in the DSP, the final filter's
group delay must be taken into account when calculating overall
group delay.

DESIGN CONSIDERATIONS

Analog Inputs

The AD73360L features six signal conditioning inputs. Each
signal conditioning block allows the AD73360L to be used with
either a single-ended or differential signal. The applied signal
can also be inverted internally by the AD73360L if required.
The analog input signal to the AD73360L can be dc-coupled,
provided that the dc bias level of the input signal is the same as
the internal reference level (REFOUT). Figure 24 shows the
recommended differential input circuit for the AD73360L. The
circuit of Figure 24 implements first-order low-pass filters
with a 3 dB point at 34 kHz; these are the only filters that must
be implemented external to the AD73360L to prevent aliasing
of the sampled signal. Since the ADC uses a highly oversampled
approach that transfers the bulk of the antialiasing filtering into
the digital domain, the off-chip antialiasing filter need only be of
a low order. It is recommended that for optimum performance the
capacitors used for the antialiasing filter be of high-quality
dielectric (NPO).
DMCLK = MCLK
REDUCED
DMCLK
32 40 48 56 64
100
VIN 100
0.047 F
TO INPUT BIAS
CIRCUITRY
Figure 24. Example Circuit for Differential Input
(DC Coupling)
The AD73360L's on-chip 38 dB preamplifier can be enabled
when there is not enough gain in the input circuit; the preampli-
fier is configured by bits IGS0–2 of CRD. The total gain must be
configured to ensure that a full-scale input signal produces a
signal level at the input to the sigma-delta modulator of the
ADC that does not exceed the maximum input range.
The dc biasing of the analog input signal is accomplished with
an on-chip voltage reference. If the input signal is not biased at
the internal reference level (via REFOUT), then it must be
ac-coupled with external coupling capacitors. CIN should be
0.1 µF or larger. The dc biasing of the input can then be accom-
plished using resistors to REFOUT as in Figure 25.
CIN
100
VIN
CIN
100
0.047 F
TO INPUT BIAS
CIRCUITRY
Figure 25. Example Circuit for Differential Input
(AC Coupling)
Figures 26 and 27 detail ac- and dc-coupled input circuits for
single-ended operation respectively.
CIN
100
VIN
0.047 F
Figure 26. Example Circuit for Single-Ended Input
(AC Coupling)
100
VIN
0.047 F
Figure 27. Example Circuit for Differential Input
(DC Coupling)
–22–
VINPx
VINNx
0.047 F
REFOUT
VOLTAGE
REFERENCE
REFCAP
0.1 F
VINPx
10k
VINNx
10k
0.047 F
REFOUT
VOLTAGE
REFERENCE
REFCAP
0.1 F
VINPx
10k
VINNx
REFOUT
VOLTAGE
REFERENCE
REFCAP
0.1 F
VINPx
VINNx
REFOUT
VOLTAGE
REFERENCE
REFCAP
0.1 F
REV. 0