Datasheet
Table 18-5. Differential Sampling Pairs (continued)
Analog InputsDifferential Pair
14 and 157
16 and 178
18 and 199
20 and 2110
22 and 2311
The voltage sampled in differential mode is the difference between the odd and even channels:
■ Input Positive Voltage: VIN+ = V
IN_EVEN
(even channel)
■ Input Negative Voltage: VIN- = V
IN_ODD
(odd channel)
The input differential voltage is defined as: VIN
D
= VIN+ - VIN-, therefore:
■ If VIN
D
= 0, then the conversion result = 0x800
■ If VIN
D
> 0, then the conversion result > 0x800 (range is 0x800–0xFFF)
■ If VIN
D
< 0, then the conversion result < 0x800 (range is 0–0x800)
When using differential sampling, the following definitions are relevant:
■ Input Common Mode Voltage: VIN
CM
= (VIN+ + VIN-) / 2
■ Reference Positive Voltage: VREFP
■ Reference Negative Voltage: VREFN
■ Reference Differential Voltage: VREF
D
= VREFP - VREFN
■ Reference Common Mode Voltage: VREF
CM
= (VREFP + VREFN) / 2
The following conditions provide optimal results in differential mode:
■ Both V
IN_EVEN
and V
IN_ODD
must be in the range of (VREFP to VREFN) for a valid conversion
result
■ The maximum possible differential input swing, or the maximum differential range, is: -VREF
D
to
+VREF
D
, so the maximum peak-to-peak input differential signal is (+VREF
D
- -VREF
D
) = 2 *
VREF
D
= 2 * (VREFP - VREFN)
■ In order to take advantage of the maximum possible differential input swing, VIN
CM
should be
very close to VREF
CM
, see Table 32-44 on page 2136.
If VIN
CM
is not equal to VREF
CM
, the differential input signal may clip at either maximum or minimum
voltage, because either single ended input can never be larger than VREFP or smaller than VREFN,
and it is not possible to achieve full swing. Thus any difference in common mode between the input
voltage and the reference voltage limits the differential dynamic range of the ADC.
Because the maximum peak-to-peak differential signal voltage is 2 * (VREFP - VREFN), the ADC
codes are interpreted as:
December 13, 20131214
Texas Instruments-Advance Information
Analog-to-Digital Converter (ADC)