ML2280, ML2283
ANALOG INPUTS AND SAMPLE/HOLD
An important feature of the ML2280 and ML2283 is that
they can be located at the source of the analog signal and
then communicate with a controlling µP with just a few
wires. This avoids bussing the analog inputs long distances
and thus reduces noise pickup on these analog lines.
However, in some cases, the analog inputs have a large
common mode voltage or even some noise present along
with the valid analog signal.
The differential input of these converters reduces the effects
of common mode input noise. Thus, if a common mode
voltage is present on both “+” and “–” inputs, such as 60Hz,
the converter will reject this common mode voltage since it
only converts the difference between “+” and “–” inputs.
The ML2280 and ML2283 have a true sample and hold
circuit which samples both “+” and “–” inputs
simultaneously. This simultaneous sampling with a true S/H
will give common mode rejection and AC linearity
performance that is superior to devices where the two input
terminals are not sampled at the same instant and where
true sample and hold capability does not exist. Thus, these
A/D converters can reject AC common mode signals from
DC-50kHz as well as maintain linearity for signals from DC-
50kHz.
The signal at the analog input is sampled during the interval
when the sampling switch is closed prior to conversion start.
The sampling window (S/H acquisition time) is 1/2 CLK
period wide and occurs 1/2 CLK period before DO goes
from high impedance to active low state. When the
sampling switch closes at the start of the S/H acquisition
time, 8pF of capacitance is thrown onto the analog input. 1/
2 CLK period later, the sampling switch is opened and the
signal present at the analog input is stored. Any error on the
analog input at the end of the S/H acquisition time will
cause additional conversion error. Care should be taken to
allow adequate charging or settling time from the source.
If more charging or settling time is needed to reduce these
analog input errors, a longer CLK period can be used.
For latchup immunity each analog input has dual diodes to
the supply rails, and a minimum of ±25mA (±100mA
typically) can be injected into each analog input without
causing latchup.
ZERO ERROR ADJUSTMENT
The zero of the A/D does not require adjustment. If the
minimum analog input voltage value, V
IN MIN
is not ground,
a zero offset can be done. The converter can be made to
output 00000000 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 difference between the actual DC input
voltage which is necessary to just cause an output digital
code transition from 00000000 to 00000001 and the ideal
1/2 LSB value (1/2 LSB = 9.8mV for V
REF
= 5.000V
DC
).
FULL-SCALE ADJUSTMENT
The full-scale adjustment can be made by applying a
differential input voltage which is 1-1/2 LSB down from
the desired analog full-scale voltage range and then
adjusting the magnitude of the V
REF
input or V
CC
for a
digital output code which is just changing from 11111110
to 11111111.
ADJUSTMENT 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,
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 00000000
to 00000001 code transition.
The full-scale adjustment should be made by forcing a
voltage to the V
IN
+ input which is given be:
(V
−
V
MIN
)
V
IN
+
fs adjust
=
V
MAX
−
1.5
×
MAX
256
where
V
MAX
= high end of the analog input range
V
MIN
= low end (offset zero) of the analog range
The V
REF
or V
CC
voltage is then adjusted to provide a
code change from 11111110 to 11111111.
12
MICRO-LINEAR [ MICRO LINEAR CORPORATION ]
