Datasheet

ADS8634
ADS8638
www.ti.com SBAS541A –MAY 2011– REVISED AUGUST 2011
The source impedance (R
SOURCE
+ R
1
) combined with (C
BYPASS
+ C
SAMPLE
) acts as a low-pass filter with
Equation 11:
Filter Time Constant = (R
SOURCE
+ R
1
) × (C
BYPASS
+ C
SAMPLE
)
Where:
C
SAMPLE
is the internal sampling capacitance of the ADC (equal to 32pF). (11)
Table 17 lists the recommended bypass capacitor values and the filter-time constant for different source
resistances. It is recommended to use a bypass capacitor with a minimum value of 100pF. Table 17 assumes R
1
= 20Ω; however, depending on the application, R
1
can be bypassed (by shorting R
1
) and the extra 20Ω margin
can be used for source resistance.
Table 17. Filter-Time Constant versus Source Resistance
APPROXIMATE C
BYPASS
FILTER TIME
R
SOURCE
(Ω) R
SOURCE
+ R
1
(pF) C
BYPASS
+ C
SAMPLE
(pF) CONSTANT (ns)
0 20 1200 1208 25
32 52 470 478 25
90 110 220 228 25
210 230 100 108 25
500 520 100 108 56
1000 1020 100 108 110
5000 5020 100 108 542
Typically, the settling resolution is selected as (ADC resolution + 2). For the ADS8634/8 (12-bit), the ideal settling
resolution is 14. Using Equation 6 and Equation 7, the sampling time can be easily determined for a given source
impedance. For source impedances greater than 210Ω, the filter-time constant continues to increase beyond the
25ns required for a 250ns sampling time. This incrementation increases the minimum permissible sampling time
for 12-bit settling and the device must be operated at a lower sampling rate.
The device sampling rate can be maximized by using a 20MHz clock for even lower throughputs. Table 18
shows typical calculations for the ADS8634/8 (12-bit).
Table 18. Sampling Frequency versus Source Impedance for the ADS8634/8
SAMPLING TIME, CONVERSION TIME, CYCLE TIME, SAMPLING RATE
R
SOURCE
(Ω) C
BYPASS
(pF) t
ACQ
(ns) t
CONV
(ns) t
ACQ
+ t
CONV
(ns) (MSPS)
750
210 100 250 1000 1
(with 20MHz clock)
750
500 100 545 1295 0.8
(with 20MHz clock)
750
1000 100 1070 1820 0.5
(with 20MHz clock)
750
5000 100 5260 6010 0.2
(with 20MHz clock)
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