Sensoray 421 - Manual (Page 16)

Page 14 Sensoray Model 421 Instruction Manual
A/D Timing Diagram
Parameter Symbol Minimum Maximum
Conversion time t
CONV
10 us
READLO to READHI delay t
DELAY
1100 ns
START*
READLO*
READHI*
BZ
Timing Diagram Signals
Name Description
START* A/D start strobe. Coincident
with host CPU write strobe to
port ADSTART.
BZ “A/D busy” flag -- BZ bit as
read by host from STATUS port.
READLO* A/D LSB read strobe. Coinci-
dent with host read strobe from
port ADLSB.
READHI* A/D MSB read strobe. Coinci-
dent with host read strobe from
port ADMSB.
t
CONV
t
DELAY
Gain Programming
A/D front-end gain may be set to any desired value
greater than or equal to unity via selection of a gain
programming resistor. Note that all analog input
channels have the same gain. Changing the front-end
gain affects all channels. As shipped from the
factory, the front-end gain is set to unity.
The front-end gain may be programmed by installing
a resistor at position R1. For any arbitrary gain R1
can be calculated using the formula
Note that for Gain = 1, R1 must be an open circuit
(default as shipped from the factory). To minimize
gain drift, R1 should have a low temperature
coefficient — 50 ppm/C or better for the best
performance.
R1
49400
Gain 1 –
------------------- =
The table below shows required values of R1 for
various gains using standard 1% resistor values. The
listed gain values are nominal and may be trimmed to
exact values as described in the next section.
R1
(ohms)
Gain Input Range
Unipolar Bipolar
Open 1 0 to +10V -5 to +5V
49.9K 2 0 to +5V -2.5 to +2.5V
12.4K 5 0 to +2V -1V to +1V
5.49K 10 0 to +1V -500 to +500mV
2.61K 20 0 to +500mV -250 to +250mV
1.00K 50 0 to +200mV -100 to +100mV
499 100 0 to +100mV -50 to +50mV
249 200 0 to +50mV -25 to +25mV
100 500 0 to +20mV -10 to +10mV
49.9 1000 0 to +10mV -5 to +5mV
Gain Trim
Since it is unlikely that the gain setting resistor R1
will produce the exact gain value desired, a gain trim
adjustment is provided on the 421
A/D front-end gain may be trimmed by adjusting
potentiometer R2. The trimmer provides an
adjustment range of approximately plus and minus
seven percent.
Offset Trim
A/D input offset may be trimmed by adjusting
potentiometer R3. This trimmer provides an
adjustment range of approximately +/- 6 millivolts.
This adjustment may alter circuit gain. Consequently,
offset should be trimmed before adjusting gain.