Rainbow Electronics ADC1175 - Manual (Page 15)

Applications Information
(Continued)
If reference voltages are desired that are more than a few
tens of millivolts from the self-bias values, the circuit of
Figure 5 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 2.8V.
3.0 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 of 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. Lead-
less 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 ADC1175, 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.
10009215
FIGURE 5. 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.
ADC1
175
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