Data Sheet
AC-Coupling
A signal transformer or series capacitors can be used to
make an AC-coupled input network. Figure 11 shows a
recommended configuration using a transformer. Make
sure that a transformer with sufficient linearity is selected,
and that the bandwidth of the transformer is appropriate.
The bandwidth should exceed the sampling rate of the
ADC with at least a factor of 10. It is also important to
keep phase mismatch between the differential ADC inputs
small for good HD2 performance. This type of transformer
coupled input is the preferred configuration for high
frequency signals as most differential amplifiers do
not have adequate performance at high frequen-
cies. Magnetic coupling between the transformers
and PCB traces may impact channel crosstalk, and
must hence be taken into account during PCB layout.
Ω
Ω
pF
Figure 12. AC-Coupled Input
Note that startup time from Sleep Mode and Power Down
Mode will be affected by this filter as the time required
to charge the series capacitors is dependent on the filter
cut-off frequency.
If the input signal has a long traveling distance, and the
kick-backs from the ADC not are effectively terminated
at the signal source, the input network of Figure 13 can
be used. The configuration is designed to attenuate the
kickback from the ADC and to provide an input impedance
that looks as resistive as possible for frequencies below
Nyquist.
33Ω
RT
47Ω
33Ω
33Ω
220Ω
33Ω
120nH
1:1
Figure 11. Transformer Coupled Input
If the input signal is traveling a long physical distance
from the signal source to the transformer (for example a
long cable), kick-backs from the ADC will also travel along
this distance. If these kick-backs are not terminated prop-
erly at the source side, they are reflected and will add to
the input signal at the ADC input. This could reduce the
ADC performance. To avoid this effect, the source must
effectively terminate the ADC kick-backs, or the traveling
distance should be very short. If this problem could not be
avoided, the circuit in Figure 10 can be used.
pF
RT
68Ω
optional
120nH
Figure 13: Alternative Input Network
Values of the series inductor will however depend on board
design and conversion rate. In some instances a shunt ca-
pacitor in parallel with the termination resistor (e.g. 33pF)
may improve ADC performance further. This capacitor
attenuate the ADC kick-back even more, and minimize
the kicks traveling towards the source. However the
impedance match seen into the transformer becomes worse.
Figure 12 shows AC-coupling using capacitors. Resistors
from the CM_EXT output, RCM, should be used to bias the
differential input signals to the correct voltage. The series
capacitor, CI, form the high-pass pole with these resistors,
and the values must therefore be determined based on
the requirement to the high-pass cut-off frequency.
©2009 CADEKA Microcircuits LLC
www.cadeka.com
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