ADC12034 National Semiconductor Corporation, ADC12034 Datasheet - Page 34

• Manufacturer Part Number: ADC12034
• Description: Self-calibrating 12-bit Plus Sign Serial I/o A/d Converters With Mux And Sample/hold
• Manufacturer: National Semiconductor Corporation

www.national.com
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, 1 LSB is equal to 610 µV. Although, the ADC
is not production tested with a 2.5V reference, when V
V
than 0.1 LSB (see the curves in the Typical Electrical Char-
acteristics Section). With the ADC set to an acquisition time
An alternative method for biasing pseudo-differential opera-
tion is to use the +2.5V from the LM4040 to bias any amplifier
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
D
+
are +5.0V linearity error typically will not change more
FIGURE 12. Pseudo-Differential Biasing with the Signal Source AC Coupled Directly into the ADC
FIGURE 11. Single-Ended Biasing
A
+
and
34
of 10 clock periods, the input biasing resistor needs to be
600Ω or less. Notice though that the input coupling capacitor
needs to be made fairly large to bring down the high pass
corner. Increasing the acquisition time to 34 clock periods
(with a 5 MHz CCLK frequency) would allow the 600Ω to in-
crease 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
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.
2
to be 2k.
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