SP574B Datasheet, PDF (6/15 Pages) Sipex Corporation – 12-Bit Sampling A/D Converters

SAMPLE–AND–HOLD FUNCTION
Although there is no sample–and–hold circuit in
the classical sense, the sampling nature of the
capacitive DAC makes the SPx74B appear to have
a built–in sample–and–hold. The sample–and–
hold function of the CDAC architecture is opti-
mized to provide full Nyquist sampling at any
maximum sampling rate. Because the S/H func-
tion is included in the ADC circuitry, the majority
of the S/H specifications are included within the A/
D specifications.
Note that some system architectures may use an
external sample–and–hold. The built–in S/H func-
tion of the SPx74B will provide additional isola-
tion. Once the internal sample is taken by the
CDAC capacitance, the input of the SPx74B is
disconnected from the input. This prevents tran-
sients occurring during conversion from being
inflicted upon the attached buffer. All other 574/
674–type circuits will cause a transient load cur-
rent on the input which will upset the buffer output
and may add error to the conversion itself. In
addition, the isolation of the input after the acqui-
sition time in the SPx74B allows you the opportu-
nity to release the HOLD on an external sample–
and–hold and start it tracking the next sample. This
will increase system throughput with your existing
components.
When using an external S/H, the SPx74B acts as
any other 574–type device because the internal S/
H is transparent. The sample/hold function in the
SPx74B is inherent to the capacitor DAC struc-
ture, and its timing characteristics are determined
by the internally generated clock. However, for
multiplexer operation, the internal S/H may elimi-
nate the need for an external S/H. The operation of
the S/H function is internal to the SPx74B and is
CE
R/C
ACQUISITION
TIME
WAIT FOR
CONVERT SIGNAL
t(ACQ)
CONVERSION
WAIT FOR
BUS READ
VIN
CDAC VOLTAGE
0 VOLTS
ACQUISITION TIME =
APERTURE DELAY TIME =
0.12 x tCONVERT
Figure 1. Sample–and–Hold Function
controlled through the normal R/C control line
(refer to Figure 1). When the R/C line makes a
negative transition, the SPx74B starts the timing
of the sampling and conversion. The first two
clock cycles are allocated to signal acquisition of
the input by the CDAC (this time is defined as
t_ACQ). Following these two cycles, the input sample
is taken and held. The A/D conversion follows this
cycle with the duration controlled by the internal
clock cycle, which is determined by the specific
product model. Note that because the sample is
taken relative to the R/C transition, t_ACQ is also the
traditional “aperture delay” of this internal sample
and hold. Since t_ACQ is measured in clock cycles,
its duration will vary with the internal clock fre-
quency.
Offset, gain and linearity errors of the S/H circuit,
as well as the effects of its droop rate, are included
in the overall specs for the SPx74B.
USING THE SPX74B SERIES
Typical Interface Circuit
The SPx74B is a complete A/D converter that is
fully operational when powered up and issued a
Start Convert Signal. Only a few external compo-
nents are necessary. The SPx74B Series have four
standard input ranges: 0V to +10V, 0V to +20V,
±5V and ±10V. Figure 2 depicts a typical interface
circuit for operating the SPx74B in a unipolar
input mode. Figure 3 depicts a typical interface
circuit for operating the SPx74B in a bipolar input
mode. Further information is given in the follow-
ing sections on these connections, but first a few
considerations concerning board layout to achieve
the best operation.
For each application of this device, strict attention
must be given to power supply decoupling, board
layout (to reduce pickup between analog and digi-
tal sections), and grounding. Digital timing, cali-
bration and the analog signal source must be
considered for correct operation.
To achieve specified accuracy, a double–sided
printed circuit board with a copper ground plane
on the component side is recommended. Keep
analog signal traces away from digital lines. It is
best to lay the PC board out such that there is an
analog section and a digital section with a single
point ground connection between the two through
an RF bead. If this is not possible, run analog
SP574B/674B/1674B/774B
12–Bit Sampling A/D Converters
6
© Copyright 2000 Sipex Corporation