DDC101 Datasheet, PDF (25/29 Pages) Burr-Brown (TI) – 20-BIT ANALOG-TO-DIGITAL CONVERTER

Aquisition
Time, K
X
N
Oversampled
Initial Data Point
Time
Clock
Cycles
M
Tracking Interval
Final Data
Point Start
M
Digital
Output
Measurement Time
DDC101 digital output is precise integration of input during measurement time.
FIGURE 25. DDC101 Equivalent Integrator Output for Single Integration with CDS.
Oversampled
X
Final Data Point
FUNCTION
USER SETTING
(Clock Cycles)
MEASUREMENT
TIME
(Calculated)
Integration Time (T_INT)
Acquisition Time K
(T_ACQ)
16
Initial Data Point
Samples, M
128
Measurement Time
Final Data Point
Samples, M
128
1ms
8µs
64µs
64µs
928µs
TABLE XVI. Measurement Time with CDS.
FUNCTION
USER SETTING
(Clock Cycles)
MEASUREMENT
TIME
(Calculated)
Integration Time (T_INT)
Acquisition Time, K
(T_ACQ) “No CDS”
Initial Data Point
Samples
Measurement Time
Final Data Point
Samples, M
1
None
128
1ms
0.5µs
0µs
64µs
967.5µs
TABLE XVII. Measurement Time without CDS.
below: one with Correlated Double Sampling (CDS) and the
other without CDS. Each example assumes that the recom-
mended system clock frequency of 2MHz is used and that
the time between “Final Data point Start” commands, (the
integration time, T_INT) is 1ms.
Example with CDS. The Measurement Time with CDS is
calculated as the Integration Time (T_INT) of 1ms less T_ACQ
and T_OS. T_OS, the oversampling time, is 1/2 of the Initial Data
Point time plus 1/2 the Final Data Point time since each
group of samples is averaged with the result at the midpoint
of each sample group.
Therefore, the Measurement Time = 1ms –(8 + 32 + 32)µs
= 928µs.
Example without CDS. The Measurement Time without
CDS is calculated as the Total Integration Time (T_INT) of
1ms less T_ACQ and T_OS. T_OS, the oversampling time, is 1/2 of
the Final Data Point time since this group of samples is
averaged with the result at the midpoint of the sample group.
Therefore, the Measurement Time = 1ms –(0.5 + 32)µs
= 967.5µs.
Input Current Calculation
The following formula calculates the input current from the
actual DDC output:
With CDS:
i
=
500 pC
•
 ^{DDC output}
 2²⁰


T _MEAS
i=
500
pC
•


DDC output
2 ²⁰


T_INT – K clock periods – M clock periods
Without CDS:
i
=
500 pC
•
 ^{DDC output}
 2²⁰


T _MEAS
i=
500
pC
•


DDC output
2 ²⁰


T_INT – K clock periods – M / 2 clock periods
®
25
DDC101