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Functional Description
712
SLAU723A – October 2017 – Revised October 2018
Copyright © 2017–2018, Texas Instruments Incorporated
Analog-to-Digital Converter (ADC)
10.3.5 Differential Sampling
In addition to traditional single-ended sampling, the ADC module supports differential sampling of two
analog input channels. To enable differential sampling, software must set the Dn bit in the ADCSSCTL0n
register in a step's configuration nibble.
When a sequence step is configured for differential sampling, the input pair to sample must be configured
in the ADCSSMUXn register. Differential pair 0 samples analog inputs 0 and 1; differential pair 1 samples
analog inputs 2 and 3; and so on (see
). The ADC does not support other differential pairings
such as analog input 0 with analog input 3.
Table 10-5. Differential Sampling Pairs
Differential Pair
Analog Inputs
0
0 and 1
1
2 and 3
2
4 and 5
3
6 and 7
4
8 and 9
5
10 and 11
6
12 and 13
7
14 and 15
8
16 and 17
9
18 and 19
10
20 and 21
11
22 and 23
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 to 0xFFF)
•
If VIN
D
< 0, then the conversion result < 0x800 (range is 0 to 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)
(5)
•
To take advantage of the maximum possible differential input swing, VIN
CM
should be very close to
VREF
CM
(see the device-specific data sheet).
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.