CROSSTALK
minimal requirement for the drive capability on the signal
conditioning preceding the A/D. Any op amp sufficient for
the signal in an application will be sufficient to drive the
ADS7824.
With a full-scale 1kHz input signal, worst case crosstalk on
the ADS7824 is better than –95dB. This should be adequate
for even the most demanding applications. However, if
crosstalk is a concern, the following items should be kept in
mind:
The resistive front end of the ADS7824 also provides a
guaranteed ±15V overvoltage protection. In most cases, this
eliminates the need for external over voltage protection
circuitry.
The worst case crosstalk is generally from Channel 3 to 2. In
addition, crosstalk from Channel 3 to any other channel is
worse than from those channels to Channel 3. The reason for
this is that channel three is nearer to the reference on the
ADS7824. This allows two coupling modes: channel-to-
channel and Channel 3 to the reference. In general, when
crosstalk is a concern, avoid placing signals with higher
frequency components on Channel 3.
INTERMEDIATE LATCHES
The ADS7824 does have tri-state outputs for the parallel
port, but intermediate latches should be used if the bus will
be active during conversions. If the bus is not active during
conversions, the tri-state outputs can be used to isolate the
A/D from other peripherals on the same bus.
If a particular channel should be as immune as possible from
crosstalk, Channel 0 would be the best channel for the signal
and Channel 1 should have the signal with the lowest
frequency content. If two signals are to have as little crosstalk
as possible, they should be placed on Channel 0 and Channel
2 with lower frequency, less-sensitive inputs on the other
channels.
Intermediate latches are beneficial on any monolithic A/D
converter. The ADS7824 has an internal LSB size of 610µV.
Transients from fast switching signals on the parallel port,
even when the A/D is tri-stated, can be coupled through the
substrate to the analog circuitry causing degradation of
converter performance. The effect of this phenomenon will
be more obvious when using the pin-compatible ADS7825
or any of the other 16-bit converters in the ADS Family. This
is due to the smaller LSB size of 38µV.
SIGNAL CONDITIONING
The FET switches used for the sample hold on many CMOS
A/D converters release a significant amount of charge injec-
tion which can cause the driving op amp to oscillate. The
amount of charge injection due to the sampling FET switch
on the ADS7824 is approximately 5-10% of the amount on
similar ADCs with the charge redistribution DAC (CDAC)
architecture. There is also a resistive front end which attenu-
ates any charge which is released. The end result is a
For an ADS7824 with proper layout, grounding, and bypass-
ing; the effect should only be a few tenths of an LSB at the
most. In those cases where this is not true, it is possible for
the conversion results to exhibit random errors of many
LSBs. Poor grounding, poor bypassing, and high-speed
digital signals will increase the magnitude of the errors.
®
ADS7824
16
BB [ BURR-BROWN CORPORATION ]
