Datasheet

25
DDC101
®
FIGURE 25. DDC101 Equivalent Integrator Output for Single Integration with CDS.
Input Current Calculation
The following formula calculates the input current from the
actual DDC output:
With CDS:
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.
USER SETTING MEASUREMENT
FUNCTION (Clock Cycles) TIME (Calculated)
Integration Time (T
INT
) 1ms
Acquisition Time K
(T
ACQ
)168µs
Initial Data Point
Samples, M 128 64µs
Measurement Time 928µs
Final Data Point
Samples, M 128 64µs
TABLE XVI. Measurement Time with CDS.
USER SETTING MEASUREMENT
FUNCTION (Clock Cycles) TIME (Calculated)
Integration Time (T
INT
) 1ms
Acquisition Time, K
(T
ACQ
) “No CDS” 1 0.5µs
Initial Data Point
Samples None 0µs
Measurement Time 967.5µs
Final Data Point
Samples, M 128 64µs
TABLE XVII. Measurement Time without CDS.
i =
500pC •
DDCoutput
2
20






T
MEAS
i =
500pC •
DDCoutput
2
20






T
MEAS
i =
500pC •
DDCoutput
2
20






T
INT
– K clock periods – M / 2 clock periods
i =
500pC •
DDCoutput
2
20






T
INT
– K clock periods – M clock periods
X
X
Measurement Time
Digital
Output
M
Aquisition
Time, K
Oversampled
Initial Data Point
N
Tracking Interval
Time
Clock
Cycles
Final Data
Point Start
M
Oversampled
Final Data Point
DDC101 digital output is precise integration of input during measurement time.