Rainbow Electronics ADC08832 - Manual (Page 14)

Functional Description
(Continued)
5.0 THE ANALOG INPUTS
The most important feature of these converters is that they
can be located right at the analog signal source and through
just a few wires can communicate with a controlling proces-
sor with a highly noise immune serial bit stream. This in itself
greatly minimizes circuitry to maintain analog signal accu-
racy which otherwise is most susceptible to noise pickup.
However, a few words are in order with regard to the analog
inputs should the input be noisy to begin with or possibly
riding on a large common-mode voltage.
The differential input of these converters actually reduces
the effects of common-mode input noise, a signal common
to both selected “+” and “−” inputs for a conversion (60 Hz is
most typical). The time interval between sampling the “+” in-
put and then the “−” input is
1 ⁄ 2 of a clock period. The change
in the common-mode voltage during this short time interval
can
cause conversion errors. For a sinusoidal
common-mode signal this error is:
where f
CM
is the frequency of the common-mode signal,
V
PEAK
is its peak voltage value
and f
CLK
is the A/D clock frequency.
For a 60Hz common-mode signal to generate a 1 ⁄ 4 LSB error
( ) 5mV) with the converter running at 250kHz, its peak value
would have to be 6.63V which would be larger than allowed
as it exceeds the maximum analog input limits.
Source resistance limitation is important with regard to the
DC leakage currents of the input multiplexer. Bypass capaci-
tors should not be used if the source resistance is greater
than 1k
Ω . The worst-case leakage current of ± 1µA over tem-
perature will create a 1mV input error with a 1k Ω source re-
sistance. An op amp RC active low pass filter can provide
both impedance buffering and noise filtering should a high
impedance signal source be required.
5.1 Sample and Hold
The ADC08831/2 provide a built-in sample-and-hold to ac-
quire the input signal. The sample and hold can sample input
signals in either single-ended or pseudo differential mode.
5.2 Input Op Amps
When driving the analog inputs with an op amp it is important
that the op amp settle within the allowed time. To achieve the
full sampling rate, the analog input should be driven with a
low impedance source (100
Ω ) or a high-speed op amp such
as the LM6142. Higher impedance sources or slower op
amps can easily be accommodated by allowing more time
for the analog input to settle.
5.3 Source Resistance
The analog inputs of the ADC08831/2 look like a 13pF ca-
pacitor (C
IN
) in series with 300 Ω resistor (Ron). C
IN
gets
switched between the selected “+” and “−” inputs during
each conversion cycle. Large external source resistors will
slow the settling of the inputs. It is important that the overall
RC time constants be short enough to allow the analog input
to completely settle.
5.4 Board layout Consideration, grounding and
bypassing:
The ADC08831/2 are easy to use with some board layout
consideration. They should be used with an analog ground
plane and single-point grounding techniques. The GND pin
should be tied directly to the ground plane.
The supply pin should be bypassed to the ground plane with
a surface mount or ceramic capacitor with leads as short as
possible. All analog inputs should be referenced directly to
the single-point ground. Digital inputs and outputs should be
shielded from and routed away from the reference and ana-
log circuitry.
6.0 OPTIONAL ADJUSTMENTS
6.1 Zero Error
The offset of the A/D does not require adjustment. If the mini-
mum analog input voltage value, V
IN(MIN)
, is not ground a
zero offset can be done. The converter can be made to out-
put 0000 0000 digital code for this minimum input voltage by
biasing any V
IN
(−) input at this V
IN(MIN)
value. This utilizes
the differential mode operation of the A/D.
The zero error of the A/D converter relates to the location of
the first riser of the transfer function and can be measured by
grounding the V
IN
(−) input and applying a small magnitude
positive voltage to the V
IN
(+) input. Zero error is the differ-
ence between the actual DC input voltage which is neces-
sary to just cause an output digital code transition from 0000
0000 to 0000 0001 and the ideal
1 ⁄ 2 LSB value ( 1 ⁄ 2 LSB =
9.8mV for V
REF
= 5.000V
DC
).
6.2 Full Scale
The full-scale adjustment can be made by applying a differ-
ential input voltage which is 1
1 ⁄ 2 LSB down from the desired
analog full-scale voltage range and then adjusting the mag-
nitude of the V
REF
input (or V
CC
for the ADC08832) for a digi-
tal output code which is just changing from 1111 1110 to 1111
1111.
6.3 Adjusting for an Arbitrary Analog Input
Voltage Range
If the analog zero voltage of the A/D is shifted away from
ground (for example, to accommodate an analog input signal
which does not go to ground), this new zero reference
should be properly adjusted first. A V
IN
(+) voltage which
equals this desired zero reference plus 1 ⁄ 2 LSB (where the
LSB is calculated for the desired analog span, using 1 LSB =
analog span/256) is applied to selected “+” input and the
zero reference voltage at the corresponding “−” input should
then be adjusted to just obtain the 00
HEX
to 01
HEX
code tran-
sition.
The full-scale adjustment should be made [with the proper
V
IN
(−) voltage applied] by forcing a voltage to the V
IN
(+) in-
put which is given by:
where:
V
MAX
= the high end of the analog input range
and
V
MIN
= the low end (the offset zero) of the analog range.
(Both are ground referenced.)
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