pair 1 samples analog inputs 2 and 3; and so on (see Table 15-6 on page 1066). The ADC does not
support other differential pairings such as analog input 0 with analog input 3.
Table 15-6. Differential Sampling Pairs
Analog Inputs
Differential Pair
0 and 1
0
2 and 3
1
4 and 5
2
6 and 7
3
8 and 9
4
10 and 11
5
12 and 13
6
14 and 15
7
16 and 17
8
18 and 19
9
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)
June 18, 2014
1066
Texas Instruments-Production Data
Analog-to-Digital Converter (ADC)
