DDC118_16 Datasheet, PDF (9/36 Pages) Texas Instruments – Octal Current Input, 20-Bit Analog-To-Digital Converter

DDC118
www.ti.com
The digital interface of the DDC118 provides the digital
results via a synchronous serial interface consisting of
differential data clocks (DCLK and DCLK), a valid data pin
(DVALID), differential serial data output pins (DOUT and
DOUT), and differential serial data input pins (DIN and
DIN). The DDC118 contains only four A/D converters, so
the conversion process is interleaved (see Figure 2). The
integration and conversion process is fundamentally
independent of the data retrieval process. Consequently,
the CLK frequency and DCLK frequencies need not be the
same. DIN and DIN are only used when multiple
converters are cascaded, and otherwise should both be
tied to DGND.
DEVICE OPERATION
Basic Integration Cycle
The topology of the front end of the DDC118 is an analog
integrator as shown in Figure 3. In this diagram, only Input
IN1 is shown. This representation of the input stage
consists of an operational amplifier, a selectable feedback
capacitor network (C_F), and several switches that
SBAS325B − JUNE 2004 − REVISED APRIL 2009
implement the integration cycle. The timing relationships
of all of the switches shown in Figure 3 are illustrated in
Figure 4. Figure 4 is used to conceptualize the operation
of the integrator input stage of the DDC118 and should not
be used as an exact timing tool for design. See Figure 5 for
the block diagrams of the reset, integrate, wait and convert
states of the integrator section of the DDC118. This
internal switching network is controlled externally with the
convert pin (CONV), range selection pins
(RANGE0-RANGE2), and the system clock (CLK). For the
best noise performance, CONV must be synchronized
with the rising edge of CLK. It is recommended that CONV
toggle within ±10ns of the rising edge of CLK.
The noninverting inputs of the integrators are connected to
ground. Consequently, the DDC118 analog ground should
be as clean as possible. The range switches, along with
the internal and external capacitors (C_F), are shown in
parallel between the inverting input and output of the
operational amplifier. At the beginning of a conversion, the
switches S_A/D, S_INTA, S_INTB, S_REF1, S_REF2, and S_RESET
are set (see Figure 4).
IN1, IN2, IN5, and IN6,
Integrator A
IN1, IN2, IN5, and IN6,
Integrator B
IN3, IN4, IN7, and IN8,
Integrator A
IN3, IN4, IN7, and IN8,
Integrator B
Conversion in Progress
DVALID
Integrate
Integrate
IN1B
IN2B
IN5B
IN6B
IN3B
IN4B
IN7B
IN8B
Integrate
Integrate
IN1A
IN2A
IN5A
IN6A
IN3A
IN4A
IN7A
IN8A
Integrate
Integrate
IN1B
IN2B
IN5B
IN6B
IN3B
IN4B
IN7B
IN8B
Integrate
Integrate
IN1A
IN2A
IN5A
IN6A
IN3A
IN4A
IN7A
IN8A
Figure 2. Basic Integration and Conversion Timing for the DDC118 (continuous mode)
S_REF1
3pF
VREF
Input
Current
Photodiode
IN1
ESD
Protection
Diodes
S_INTA
S_INTB
S_RESET
50pF
25pF
12.5pF
S_REF2 S_A/D1A
Integrator A
Integrator B (same as A)
RANGE2
RANGE1
RANGE0
To Converter
Figure 3. Basic Integration Configuration for Input 1, shown with a 250pC (C_F = 62.5pF) Input Range
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