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ADC12H030 Datasheet(PDF) 31 Page - National Semiconductor (TI)

[Old version datasheet] Texas Instruments acquired National semiconductor.
Part No. ADC12H030
Description  Self-Calibrating 12-Bit Plus Sign Serial I/O A/D Converters with MUX and Sample/Hold
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Maker  NSC [National Semiconductor (TI)]
Homepage  http://www.national.com
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ADC12H030 Datasheet(HTML) 31 Page - National Semiconductor (TI)

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Application Hints (Continued)
For pseudo-differential signed operation, the biasing circuit
shown in Figure 12 shows a signal AC coupled to the ADC.
This gives a digital output range of −4096 to +4095. With a
2.5V reference, as shown, 1 LSB is equal to 610 µV. Al-
though, the ADC is not production tested with a 2.5V refer-
ence, linearity error typically will not change more than 0.1
LSB (see the curves in the Typical Electrical Characteristics
Section). With the ADC set to an acquisition time of 10 clock
periods, the input biasing resistor needs to be 600
Ω or less.
Notice though that the input coupling capacitor needs to be
made fairly large to bring down the high pass corner. In-
creasing the acquisition time to 34 clock periods (with a
5 MHz CCLK frequency) would allow the 600
Ω to increase to
6k, which with a 1 µF coupling capacitor would set the high
pass corner at 26 Hz. Increasing R, to 6k would allow R
2 to
be 2k.
An alternative method for biasing pseudo-differential opera-
tion is to use the +2.5V from the LM4040 to bias any ampli-
fier circuits driving the ADC as shown in Figure 13. The value
of the resistor pull-up biasing the LM4040-2.5 will depend
upon the current required by the op amp biasing circuitry.
In the circuit of Figure 13 some voltage range is lost since
the amplifier will not be able to swing to +5V and GND with
a single +5V supply. Using an adjustable version of the
LM4041 to set the full scale voltage at exactly 2.048V and a
lower grade LM4040D-2.5 to bias up everything to 2.5V as
shown in Figure 14 will allow the use of all the ADC’s digital
output range of −4096 to +4095 while leaving plenty of head
room for the amplifier.
Fully differential operation is shown in Figure 15. One LSB
for this case is equal to (4.1V/4096) = 1 mV.
01135447
FIGURE 12. Pseudo-Differential Biasing with the Signal Source AC Coupled Directly into the ADC
www.national.com
31


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