adc12eu050eb National Semiconductor Corporation, adc12eu050eb Datasheet - Page 26

• Manufacturer Part Number: adc12eu050eb
• Description: Adc12eu050 Ultra-low Power, Octal, 12-bit, 40-50 Msps Sigma-delta Analog-to-digital Converter
• Manufacturer: National Semiconductor Corporation

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Input Coupling and Common Mode
The ADC12EU050 internally generates a common mode of
0.62V. It is possible to provide input signals with other com-
mon modes however, the full scale input range of the ADC
must be kept in mind.
For this reason, it is recommended that the device inputs are
AC coupled. The recommended capacitor value is 100nF.
External Series Resistance
R
Due to manufacturing the value of this resistance can vary by
up to 15%. This is not important for the operation of the ADC,
since the ADC depends only on internal resistors being
matched, but it should be taken into account when performing
calculations.
ADC
, the input resistance of the ADC, is nominally 1.3kΩ.
FIGURE 15. External Series Resistance
FIGURE 14. Single Ended Input
26
Due to the purely resistive input circuit of the sigma delta ar-
chitecture, the ADC12EU050 allows the user to scale down
large input signals by adding external series resistors. The
gain achieved by adding external resistors can be calculated
as a simple voltage divider, as follows:
The diagram below shows this configuration, and defines the
values in the equation above.
2.5 ADC OUTPUT CONSIDERATIONS
The ADC12EU050 offers a variety of output settings in order
to cater for different system design and integration needs.
Output Driver Voltage, VDR
The ADC output driver voltage, VDR, can be set between
1.2V and 1.8V. A VDR of 1.2V will offer the lowest power
consumption. Because VDR can be varied, the ADC12EU050
provides, via the SPI registers, the ability to adjust the output
common mode voltage.
V
FS
/V
IN
= 20 * log (R
ADC
/( R
ADC
+ R
30051120
EXT
30051118
)) dB