CDK8307 datasheet(26/29 Pages) CADEKA

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PRELIMINARY Data Sheet

AC-Coupling

A signal transformer or series capacitors can be used

to make an AC-coupled input network. Figure 8 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.

Figure 10. 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.

Figure 11 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.

Figure 11. 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 12 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.

Figure 12: Alternative Input Network

Values of the series inductor will however depend on

board design and conversion rate. In some instances a

shunt capacitor 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.

120nH

220

optional

1:1

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