ADC1175CIMTC National Semiconductor, ADC1175CIMTC Datasheet - Page 13

• Manufacturer Part Number: ADC1175CIMTC
• Description: 8-Bit/ 20MHz/ 60mW A/D Converter
• Manufacturer: National Semiconductor

Applications Information
If reference voltages are desired that are more than a few
tens of millivolts from the self-bias values, the circuit of Fig-
ure 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
V
supply voltage, and V
and 1.0V below V
accurate conversions, the total reference voltage range (V
- V
about 2.8V.
3.0 Power Supply Considerations
Many A/D converters draw sufficient transient current to cor-
rupt 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
FIGURE 5. Driving the reference to force desired values requires driving with a low impedance source, provided by
RT
RB
can be anywhere between V
) should be a minimum of 1.0V and a maximum of
RTS
RT
and V
. To minimize noise effects and ensure
RB
can be anywhere between ground
RBS
pins are left floating.
the transistors. Note that pins 16 and 22 are not connected.
RB
+ 1.0V and the analog
(Continued)
RT
13
possible to the converter’s power supply pins. Leadless chip
capacitors are preferred because they have low lead induc-
tance.
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.
As is the case with all high speed converters, the ADC1175
should be assumed to have little power supply rejection, es-
pecially when self-biasing is used by connecting V
V
No pin should ever have a voltage on it that is in excess of
the supply voltages or below ground, not even on a trasient
RTS
together.
DS100092-15
www.national.com
RT
and