Datasheet

ADS7800
10
SBAS001A
www.ti.com
FIGURE 10. ±10V Range With External Trims.
converter if at all possible. A ground plane is usually the best
solution for preserving dynamic performance and reducing
noise coupling into sensitive converter circuits. Where any
compromises must be made, the common return of the
analog input signal should be referenced to pin 4, AGND,
on the ADS7800, which prevents any voltage drops that
might occur in the power supply common returns from
appearing in series with the input signal.
Coupling between analog input and digital lines should be
minimized by careful layout. For instance, if the lines must
cross, they should do so at right angles. Parallel analog and
digital lines should be separated from each other by a pattern
connected to common.
If external full scale and offset potentiometers are used, the
potentiometers and related resistors should be located as
close to the ADS7800 as possible.
REFERENCE BYPASS
Pin 3 (REF) should be bypassed with a 22µF to 47µF
tantalum capacitor. A rated working voltage of 2V or more
is acceptable here. This pin is used to enhance the system
accuracy of the internal reference circuit, and is not
recommended for driving external signals. If there are
important system reasons for using the ADS7800 reference
externally, the output of pin 3 must be appropriately
buffered.
“HOT SOCKET” PRECAUTION
Two separate +5V V
S
pins, 23 and 24, are used to minimize
noise caused by digital transients. If one pin is powered and
the other is not, the ADS7800 may “Latch Up” and draw
excessive current. In normal operation, this is not a problem
because both pins will be soldered together. However,
during evaluation, incoming inspection, repair, etc., where
the potential of a “Hot Socket” exists, care should be taken
to power the ADS7800 only after it has been socketed.
FIGURE 9. Read Cycle Timing.
MINIMIZING “GLITCHES”
Coupling of external transients into an A/D converter can
cause errors which are difficult to debug. In addition to the
discussions earlier on layout considerations for supplies,
bypassing and grounding, there are several other useful
steps that can be taken to get the best analog performance
out of a system using the ADS7800. These potential system
problem sources are particularly important to consider when
developing a new system, and looking for the causes of
errors in breadboards.
First, care should be taken to avoid glitches during critical
times in the sampling and conversion process. Since the
ADS7800 has an internal sample/hold function, the signal
that puts it into the hold state (R/C going LOW) is critical, as
it would be on any sample/hold amplifier. The R/C falling
edge should be sharp and have minimal ringing, especially
during the 20ns after it falls.
Although not normally required, it is also good practice to
avoid glitching the ADS7800 while bit decisions are being
made. Since the above discussion calls for a fast, clean rise
and fall on R/C, it makes sense to keep the rising edge of the
convert pulse outside the time when bit decisions are being
made. In other words, the convert pulse should either be
short (under 100ns so that it transitions before the MSB
decision), or relatively long (over 2.75µs to transition after
the LSB decision).
FIGURE 11. ±5V Range With External Trims.
Data Valid
t
DB
t
HL
t
HDR
&
t
DD
CS
R/C
HBE
BUSY
DB11-DB0
1
2
3
4
5
6
7
ADS7800
+5V
100Ω
External
Gain Adjust
±10V
Input
10k
–15V
10kΩ
6.65kΩ
49.9Ω
R
2
R
1
Bipolar
Zero
Adjust
1
2
3
4
5
6
7
ADS7800
+5V
100Ω
±5V
Input
–15V
10kΩ
10kΩ
301Ω
30.1kΩ
External
Gain Adjust
R
2
R
1
Bipolar
Zero
Adjust