Datasheet
ADC1173
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SNAS025F –FEBRUARY 1999–REVISED APRIL 2013
Driving the reference to force desired values requires driving with a low impedance source, provided by the
transistors. Note that pins 16 and 22 are not connected.
Figure 28. Driving with a low impedance source
If reference voltages are desired that are more than a few tens of millivolts from the self-bias values, the circuit of
will allow forcing the reference voltages to whatever levels are desired. This circuit provides the best
performance because of the low source impedance of the transistors. Note that the V
RTS
and V
RBS
pins are left
floating.
V
RT
can be anywhere between V
RB
+ 1.0V and the analog supply voltage, and V
RB
can be anywhere between
ground and 1.0V below V
RT
. To minimize noise effects and ensure accurate conversions, the total reference
voltage range (V
RT
- V
RB
) should be a minimum of 1.0V and a maximum of about V
A
. Best performance can be
realized with V
RT
= 1.56 and V
RB
= 0.36V. If V
RB
is not required to be below about +700mV, the -5V points in can
be returned to ground and the negative supply eliminated.
POWER SUPPLY CONSIDERATIONS
Many A/D converters draw sufficient transient current to corrupt their own power supplies if not adequately
bypassed. A 10µF tantalum or aluminum electrolytic capacitor should be placed within an inch (2.5 centimeters)
of the A/D power pins, with a 0.1 µF ceramic chip capacitor placed as close as possible to the converter's power
supply pins. Leadless chip capacitors are preferred because they have low lead inductance.
While a single voltage source should be used for the analog and digital supplies of the ADC1173, these supply
pins should be well isolated from each other to prevent any digital noise from being coupled to the analog power
pins. A 47 Ohm resistor is recommend between the analog and digital supply lines, with a ceramic capacitor
close to the analog supply pin. Avoid inductive components in the analog supply line.
The converter digital supply should not be the supply that is used for other digital circuitry on the board. It should
be the same supply used for the A/D analog supply.
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