Datasheet

ADS8634
ADS8638
SBAS541A –MAY 2011– REVISED AUGUST 2011 www.ti.com
Values of m
REF
and C
REF
for any reference voltage other than 2.5V can be calculated using Equation 3 and
Equation 4:
m
REF
= m
REF_2.5
× 2.5/V
REF
(3)
C
REF
= (C
REF_2.5
– 3584) × 2.5/V
REF
+ 3584 (4)
For example, at a 2V reference:
m
REF_2
= 0.47 × 2.5/2 = 0.59 and
C
REF_2
= (3777.2 – 3584) × 2.5/2 + 3584 = 3825.5
For the reference voltage used, Equation 2 can be rewritted using m
REF
and C
REF
as calculated in Equation 3
and Equation 4.
Table 3 can be used as quick reference for temperature sensor transfer function at typical reference values.
Table 3. Temperature Sensor Transfer Function at Typical Reference Values
REFERENCE VOLTAGE (V) TRANSFER FUNCTION
2 Device temperature in °C = (output code – 3825.5)/0.59
2.5 Device temperature in °C = (output code – 3777.2)/0.47
3 Device temperature in °C = (output code – 3745.0)/0.39
DATA FORMAT
The ADS8634/8 output 12-bits of ADC conversion results in binary format (MSB first) for all ranges, as shown in
Table 4. Figure 76 shows the ADC transfer function for bipolar signal ranges. The unipolar range output is shown
in Table 5 and Figure 77 shows the transfer function.
Table 4. Bipolar Range Ideal Output Codes
(1)
INPUT SIGNAL (AINx – AINGND)
IDEAL OUTPUT CODE
±10V RANGE (V) ±5V RANGE (V) ±2.5V RANGE (V)
≥ 10 × (2
11
– 1)/2
11(2)
≥ 5 × (2
11
– 1)/2
11
≥ 2.5 × (2
11
– 1)/2
11
FFFh
10/2
11
5/2
11
2.5/2
11
801h
0 0 0 800h
–10 /2
11
–5/2
11
–2.5/2
11
7FFh
≤ –10 × (2
11
– 1)/2
11
≤ –5 × (2
11
– 1)/2
11
≤ –2.5 × (2
11
– 1)/2
11
000h
(1) Excludes noise, offset and gain errors.
(2) LSB size for the bipolar ranges = positive (or negative) full-scale/2
11
. The ADS8634/8 offer 12-bit resolution across the entire range from
positive full-scale to negative full-scale; in other words, the resolution for half range from '0' to positive (or negative) full-scale is 11 bits.
For example, a 1LSB for a ±10V range is 10/2
11
.
Table 5. Unipolar Range Ideal Output Codes
(1)
INPUT SIGNAL (AINx – AINGND)
0V TO 10V RANGE (V) 0V TO 5V RANGE (V) IDEAL OUTPUT CODE
≥ 10 × (2
12
– 1)/2
12
≥ 5 × (2
12
– 1)/2
12
FFFh
10/2
12
5/2
12
001h
< 10/2
12
< 5/2
12
000h
(1) Excludes noise, offset and gain errors.
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