DDC101 datasheet(17/30 Pages) TI2

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DDC101

Correlated Double Sampling is implemented in the DDC101

by subtracting the Initial Data Point from the Final Data

Point. Thus, the error correction is updated automatically for

each integration. When operating in the unipolar input range,

CDS functions with either output data format—straight

binary or binary two’s complement. When operating in the

bipolar input range, CDS functions with binary two’s comple-

ment output data format only.

The errors that CDS removes are charge injection, kT/C and

DDC101 input voltage offset. These errors are very difficult

to eliminate in equivalent analog circuits. Charge injection

errors result from charge that is transferred through the reset

switch into the integration capacitor. kT/C errors are switch-

ing errors due to the noise of the resistance of the reset

switch. DDC101 voltage offset errors are due to input offset

of the input comparator. Both initial offset and offset drift

with time and temperature are corrected since the correction

is performed each integration cycle.

SINGLE CYCLE INTEGRATION

The DDC101 acquires charge (q) by integrating input cur-

rent (i) for a specific time (T). That is,

The DDC101 acquires up to 500pC of full scale charge per

integration cycle in the unipolar input range, and approxi-

mately

±250pC of full scale charge in the bipolar input

range. Therefore, for the DDC101, maximum values can be

calculated.

Unipolar Input Range

Bipolar Input Range

500pC = I

FS x TINT

±250pC = ±I

FS x TINT

Where I

FS is the full scale input current and TINT is the

integration time of the DDC101. Examples of I

FS and TINT

that equal 500pC and

±250pC are shown in the following

tables.

The maximum average input current that the DDC101 can

integrate is 7.8

µA. This results in a minimum integration

time of 64

µs for unipolar inputs and 32µs for bipolar inputs.

Further flexibility is possible with multiple integration cycles

per conversion as described in the following text.

INPUT RANGE

Unipolar Input Range

For the unipolar input range, the range of charge for each

integration cycle is from positive full scale of +500pC to a

slightly negative charge of –1/256 (approximately –0.4%) of

the positive full scale charge. This is +500pC to –1.95pC.

The negative charge measurement capability allows for low

level PC board parasitic leakages.

Bipolar Input Range

For the bipolar input range, the range of charge for each

integration cycle is from positive full scale of +250pC to

negative full scale of –251.95pC.

FIGURE 13. Close-up of Reset and Acquisition Time for

DDC101.

Signal Acquired

Reset of

Previous Integration

Ideal Integration

Actual Integration

Acquisition Time, K

FIGURE 14. Close-up of End of One Integration Cycle and

Beginning of Next.

Tracking Interval

Final

Oversampled

Data

Integration n

Integration n + 1

Initial

Oversampled

Data

Acquisition

Reset

Final Data Point Start

NON-CONTINUOUS INTEGRATION MODE

For the non-continuous integration mode, FDS controls the

start of the M final data point samples and the end of

integration as discussed above. In this mode, however, FDS

is also used to control the start of a new integration cycle

asynchronously with the end of the previous integration.

When FDS transitions to “ON”, the collection of the M final

data point samples begins. At the end of each integration, the

DDC101 automatically resets the integration capacitance. If

FDS remains “ON” past the end of integration, the DDC101

will stay in the integration reset state until FDS transitions to

“OFF”. Holding FDS “ON” past the end of integration will

also reset the DDC101’s tracking logic to zero integration

rate.

In non-continuous integration mode, the initial data point

measurement may be less accurate since the DDC101’s

internal tracking logic is reset at the beginning of the

integration and tracking may not be accurate for the initial

data point measurement. In this situation, Correlated Double

Sampling (CDS) operation may not be advantageous.

INTERNAL ERROR CORRECTION

The DDC101 uses CDS techniques to gain optimum perfor-

mance. CDS removes internal DDC101 errors which occur

for a given integration cycle such as, charge injection, kT/C,

and DDC101 offset errors. Correlated Double Sampling is

user selectable. It is recommended for most continuous

measurement applications.

= i

∫ dt

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