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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*

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.

CONV

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.

49400

Gain1–

-------------------=

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.

(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.

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