Datasheet

ADS7800
11
SBAS001A
www.ti.com
INPUT VOLTAGE RANGE AND LSB VALUES
Input Voltage Range Defined As: ±10V ±5V
Analog Input Connected to Pin 1 2
Pin Connected to GND 21
One Least Significant Bit (LSB) FSR/2
12
20V/2
12
10V/2
12
4.88mV 2.44mV
OUTPUT TRANSITION VALUES
FFE
H
to FFF
H
+Full Scale +10V–3/2LSB +5V–3/2LSB
+9.9927V +4.9963V
7FF
H
to 800
H
Mid Scale 0V–1/2LSB 0V–1/2LSB
(Bipolar Zero) –2.44mV –1.22mV
000
H
to 001
H
–Full Scale –10V+1/2LSB –5V+1/2LSB
–9.9976V –4.9988V
TABLE IV. Input Voltages, Transition Values, and LSB Values.
Next, although the data outputs are forced into a Hi-Z state
during conversion, fast bus transients can still be capaci-
tively coupled into the ADS7800. If the data bus experiences
fast transients during conversion, these transients can be
attenuated by adding a logic buffer to the data outputs. The
BUSY output can be used to enable the buffer.
Naturally, transients on the analog input signal are to be
avoided, especially at times within ±20ns of R/C going
LOW, when they may be trapped as part of the charge on the
capacitor array. This requires careful layout of the circuit in
front of the ADS7800.
Finally, in multiplexed systems, the timing on when the
multiplexer is switched may affect the analog performance
of the system. In most applications, the multiplexer can be
switched as soon as R/C goes LOW (with appropriate
delays), but this may affect the conversion if the switched
signal shows glitches or significant ringing at the ADS7800
input. Whenever possible, it is safer to wait until the
conversion is completed before switching the multiplexer.
The extremely fast acquisition time and conversion time of
the ADS7800 make this practical in many applications.