ADS8694 Datasheet, PDF (32/77 Pages) Texas Instruments – 4- and 8-Channel, Single-Supply, SAR ADCs

ADS8694, ADS8698
SBAS686 – JULY 2015
www.ti.com
8.3.10 ADC Transfer Function
The ADS8694 and ADS8698 are a family of multichannel devices that support single-ended, bipolar, and
unipolar input ranges on all input channels. The output of the devices is in straight binary format for both bipolar
and unipolar input ranges. The format for the output codes is the same across all analog channels.
The ideal transfer characteristic for each ADC channel for all input ranges is shown in Figure 69. The full-scale
range (FSR) for each input signal is equal to the difference between the positive full-scale (PFS) input voltage
and the negative full-scale (NFS) input voltage. The LSB size is equal to FSR / 2¹⁸ = FSR / 262144 because the
resolution of the ADC is 18 bits. For a reference voltage of V_REF = 4.096 V, the LSB values corresponding to the
different input ranges are listed in Table 4.
3FFFFh
20000h
00001h
1LSB
NFS
FSR/2
FSR ± 1LSB
PFS
Analog Input (AIN_nP J AIN_nGND)
Figure 69. 18-Bit ADC Transfer Function (Straight-Binary Format)
INPUT RANGE
±2.5 × V_REF
±1.25 × V_REF
±0.625 × V_REF
0 to 2.5 × V_REF
0 to 1.25 × V_REF
Table 4. ADC LSB Values for Different Input Ranges (V_REF = 4.096 V)
POSITIVE FULL-SCALE NEGATIVE FULL-SCALE FULL-SCALE RANGE
10.24 V
–10.24 V
20.48 V
5.12 V
–5.12 V
10.24 V
2.56 V
–2.56 V
5.12 V
10.24 V
0V
10.24 V
5.12 V
0V
5.12 V
LSB (µV)
78.125
39.0625
19.53125
39.0625
19.53125
32
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