DDC101 Datasheet, PDF (13/29 Pages) Burr-Brown (TI) – 20-BIT ANALOG-TO-DIGITAL CONVERTER

SECTION 5
DISCUSSION OF
SPECIFICATIONS
INPUT
The DDC101 is a charge digitizing A/D converter. Low
level current output sources, such as a photosensors, can be
directly connected to its input. The input signal can also be
a voltage connected through a user selected resistor.
CHARGE INPUT
The maximum charge that can be captured in one integration
by the DDC101 is 500pC. In the unipolar input range mode,
the maximum positive charge that can be collected in one
integration is 500pC. The DDC101 has a small negative
range in the unipolar mode of –1.95pC. This small negative
underrange is included to allow for a small amount of
leakage current from the user’s PC board and sensor. In the
bipolar input range, the maximum positive charge that can
be collected is +250pC. The maximum negative charge that
can be collected is –251.95pC.
In addition to the normal mode of one integration per
conversion, DDC101 can be configured by the user for 1 to
256 integrations per conversion. When the multiple integra-
tions per conversion mode is chosen, the DDC101 DSP
circuitry internally averages multiple integration cycles to
provide one conversion result. This result has lower noise
because it is the average of multiple integrations. In this
mode, the maximum total charge that can be captured by the
DDC101 in 256 integrations is 128,000pC.
TEST CURRENT INPUT
An internal DC test current can be connected under user
control to the DDC101’s input. The test current is nominally
100nA and will be summed with any applied external input
signal. It is derived by a resistive network from the positive
power supply. The test current is intended to test for func-
tionality only. The TEST In pin of the DDC101 controls the
current. When TEST is HIGH, the internal current source is
ON and current is flowing into the DDC101 input. When
TEST is LOW, the current source is disconnected from the
input. With TEST active, positive power supply current
increases by approximately 1mA.
FULL SCALE RANGE
The full scale range (FSR), which is referenced in the
specification table, is the difference between the positive full
scale charge and the negative full scale charge for the
DDC101 in one integration cycle. Specifications such as
noise and linearity, which are specified in percent or ppm of
FSR, are referring to a value of 500pC for both unipolar and
bipolar input ranges.
The full scale input current for a given integration time will
result in a full scale input charge. As an example for unipolar
input range, an input current of 0.5µA integrated for 1ms
will result in the full scale charge of 500pC. For voltage
inputs, the input resistor is chosen to achieve the proper full
scale input current. As an example, for a 5V full scale input,
a 10MΩ input resistor is selected to achieve a full scale input
current of 0.5µA (1ms integration time).
Noise of 1.6ppm of FSR is equal to 1.6ppm x 500pC = 0.8fC
or 1.6ppm x 0.5µA = 0.8pA or 1.6ppm x 5V = 8µV. Thus,
in this instance, noise is 1.6pA or 8µV.
For the unipolar input range, the following table shows the
full scale input current required for different integration
times to collect 500pC of charge and the equivalent current
values for 2 and 5ppm of FSR.
T_INT
50ms
5ms
1ms
500µs
100µs
I_FS
10nA
100nA
500nA
1µA
5µA
2ppm
0.02pA
0.2pA
1pA
2pA
10pA
5ppm
0.5pA
1pA
2.5pA
5pA
25pA
TABLE I. Integration Time (T_INT) and Full Scale Current (I_FS)
for Full Scale 500pC Integration.
CURRENT INPUT
The maximum average input current that can be captured by
the DDC101 is ±7.8µA. This current will result in an
integration time of 64µs for unipolar input range and 32µs
for bipolar input range. For longer integration times, the
average input current must be less.
The maximum input current is limited by the slew and
update rate of the internal tracking logic and CDAC. The
largest input current that the DDC101 can accurately track is
7.8µA. Input currents larger than 7.8µA and high speed
current input pulses can be accurately captured and digitized
by the DDC101 with an external input or sensor capacitance
on the DDC101 input. The average current during a com-
plete integration cycle cannot exceed 7.8µA. Likewise, the
total charge input must not exceed 500pC unipolar, 250pC
bipolar during the integration time.
An external user provided input capacitance, C_S, as shown in
Figure 9a, will capture the input signal charge if the input
current limit is temporarily exceeded during the integration
cycle. The DDC101 will then transfer the charge completely
to C_INT based upon conservation of charge. An additional
Voltage across input must not exceed ±2.5V.
i
Analog Input, pin 3
C_S
V
DDC101
Analog Common
External user provided capacitance, C_SOURCE, to store current pulses.
FIGURE 9a. Current Pulse Input Capture.
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13
DDC101