EVAL-ADUC831QSZ Datasheet, PDF (22/76 Pages) Analog Devices – MicroConverter®, 12-Bit ADCs and DACs with Embedded 62 kBytes Flash MCU

ADuC831
Driving the A/D Converter
The ADC incorporates a successive approximation (SAR) archi-
tecture involving a charge-sampled input stage. Figure 9 shows
the equivalent circuit of the analog input section. Each ADC
conversion is divided into two distinct phases as defined by the
position of the switches in Figure 9. During the sampling phase
(with SW1 and SW2 in the “track” position) a charge propor-
tional to the voltage on the analog input is developed across the
input sampling capacitor. During the conversion phase (with
both switches in the “hold” position) the capacitor DAC is
adjusted via internal SAR logic until the voltage on node A is
zero, indicating that the sampled charge on the input capacitor is
balanced out by the charge being output by the capacitor DAC.
The digital value finally contained in the SAR is then latched out
as the result of the ADC conversion. Control of the SAR, and
timing of acquisition and sampling modes, is handled auto-
matically by built-in ADC control logic. Acquisition and
conversion times are also fully configurable under user control.
AIN7
V_REF
AGND
ADuC831
DAC1
DAC0
TEMPERATURE MONITOR
AIN0
AGND
200⍀
CAPACITOR
DAC
TRACK sw1
HOLD
32pF
COMPARATOR
200⍀ ^{NODE A}
sw2
TRACK
HOLD
Figure 9. Internal ADC Structure
Note that whenever a new input channel is selected, a residual
charge from the 32 pF sampling capacitor places a transient on
the newly selected input. The signal source must be capable of
recovering from this transient before the sampling switches click
into “hold” mode. Delays can be inserted in software (between
channel selection and conversion request) to account for input
stage settling, but a hardware solution will alleviate this burden
from the software design task and will ultimately result in a
cleaner system implementation. One hardware solution would
be to choose a very fast settling op amp to drive each analog
input. Such an op amp would need to fully settle from a small
signal transient in less than 300 ns in order to guarantee adequate
settling under all software configurations. A better solution, recom-
mended for use with any amplifier, is shown in Figure 10.
Though at first glance the circuit in Figure 10 may look like a
simple antialiasing filter, it actually serves no such purpose since its
corner frequency is well above the Nyquist frequency, even at a
200 kHz sample rate. Though the R/C does helps to reject some
incoming high-frequency noise, its primary function is to ensure
that the transient demands of the ADC input stage are met. It
ADuC831
10⍀
0.1␮F
AIN0
Figure 10. Buffering Analog Inputs
does so by providing a capacitive bank from which the 32 pF
sampling capacitor can draw its charge. Its voltage will not
change by more than one count (1/4096) of the 12-bit trans-
fer function when the 32 pF charge from a previous channel
is dumped onto it. A larger capacitor can be used if desired,
but not a larger resistor (for reasons described below).
The Schottky diodes in Figure 10 may be necessary to limit the
voltage applied to the analog input pin as per the data sheet
absolute maximum ratings. They are not necessary if the op
amp is powered from the same supply as the ADuC831 since
in that case the op amp is unable to generate voltages above
V_DD or below ground. An op amp of some kind is necessary
unless the signal source is very low impedance to begin with.
DC leakage currents at the ADuC831’s analog inputs can
cause measurable dc errors with external source impedances
as little as 100 Ω or so. To ensure accurate ADC operation, keep
the total source impedance at each analog input less than 61 Ω.
The table below illustrates examples of how source impedance
can affect dc accuracy.
Source
Impedance
61 Ω
610 Ω
Error from 1 µA
Leakage Current
61 µV = 0.1 LSB
610 µV = 1 LSB
Error from 10 µA
Leakage Current
610 µV = 1 LSB
6.1 mV = 10 LSB
Although Figure 10 shows the op amp operating at a gain of 1,
you can, of course, configure it for any gain needed. Also, you
can just as easily use an instrumentation amplifier in its place to
condition differential signals. Use any modern amplifier that is
capable of delivering the signal (0 to V_REF) with minimal satura-
tion. Some single-supply rail-to-rail op amps that are useful for
this purpose include, but are certainly not limited to, the ones
given in Table VI. Check Analog Devices literature (CD ROM
data book, and so on) for details on these and other op amps
and instrumentation amps.
Table VI. Some Single-Supply Op Amps
Op Amp Model
OP281/OP481
OP191/OP291/OP491
OP196/OP296/OP496
OP183/OP283
OP162/OP262/OP462
AD820/AD822/AD824
AD823
Characteristics
Micropower
I/O Good up to V_DD, Low Cost
I/O to V_DD, Micropower, Low Cost
High Gain-Bandwidth Product
High GBP, Micro Package
FET Input, Low Cost
FET Input, High GBP
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REV. 0