SM73201 Datasheet, PDF (13/21 Pages) Texas Instruments – 16-Bit, 50 to 250 kSPS, Differential Input, MicroPower ADC

Both inputs should be biased at a common mode voltage
(V_CM), which will be thoroughly discussed in Section 2.3. Fig-
ure 8 shows the SM73201 being driven by a full-scale differ-
ential source.
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FIGURE 8. Differential Input
2.2 Single-Ended Input Operation
For single-ended operation, the non-inverting input (+IN) of
the SM73201 can be driven with a signal that has a peak-to-
peak range that is equal to or less than (2 x V_REF). The
inverting input (−IN) should be biased at a stable V_CM that is
halfway between these maximum and minimum values. In
order to utilize the entire dynamic range of the SM73201,
V_REF is limited to (V_A / 2). This allows +IN a maximum swing
range of ground to V_A. Figure 9 shows the SM73201 being
driven by a full-scale single-ended source.
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FIGURE 10. V_CM range for Differential Input operation
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FIGURE 9. Single-Ended Input
Since the design of the SM73201 is optimized for a differential
input, the performance degrades slightly when driven with a
single-ended input. Linearity characteristics such as INL and
DNL typically degrade by 0.1 LSB and dynamic characteris-
tics such as SINAD typically degrade by 2 dB. Note that
single-ended operation should only be used if the perfor-
mance degradation (compared with differential operation) is
acceptable.
2.3 Input Common Mode Voltage
The allowable input common mode voltage (V_CM) range de-
pends upon V_A and V_REF used for the SM73201. The ranges
of V_CM are depicted in Figure 10 and Figure 11. Note that
these figures only apply to a V_A of 5V. Equations for calculat-
ing the minimum and maximum V_CM for differential and single-
ended operations are shown in Table 1.
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FIGURE 11. V_CM range for single-ended operation
TABLE 1. Allowable V_CM Range
Input Signal Minimum V_CM
Maximum V_CM
Differential
V_REF / 2
V_A − V_REF / 2
Single-Ended
V_REF
V_A − V_REF
2.4 CMRR
By using this differential input, small signals common to both
inputs are rejected. As shown in Figure 12, noise is immune
at low frequencies where the common-mode rejection ratio
(CMRR) is 90 dB. As the frequency increases to 1 MHz, the
CMRR rolls off to 40 dB . In general, operation with a fully
differential input signal or voltage will provide better perfor-
mance than with a single-ended input. However, if desired,
the SM73201 can be presented with a single-ended input.
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