SP8121DS/02

SP8121 Monolithic, 12-Bit Data Acquisition System

© Copyright 2000 Sipex Corporation

6

SP8121

OFFSET ADJUST

100K

+15V

5K

–1.5mV to +3mV

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Figure 1. Offset Adjust

SP8121

GAIN ADJUST

125K

+15V

10K

±0.3% Trim Range

7

19K

Center pot

for zero

correction

Figure 2. Gain Adjust

The conversion cycle is started when R/C is brought

lowandmustbeheldlowforaminimumof50ns.The

R/C signal will also put the output latches in a tri-state

mode when low. Approximately 200ns after R/C is

low, STATUS will change from low to high. This

output signal will stay high while the SP8121 is

performingaconversion.Validdatawillbelatchedto

the output bus, through internal control, 500ns prior

to the STATUS line transitioning from a high to low.

READING THE DATA

Please refer to Figure 4. To read data from the

SP8121, the R/C and CE control lines are used. R/C

must be high a minimum of 50ns prior to reading

the data to allow time for the output latches to come

out of the high impedance tri-state mode. CE is used

to access the data. The first 8 MSBs will be on

pins 32 through 25, with pin 32 being the MSB.

The remaining 4 LSBs will be on pins 21 through 24

with pin 21 being the LSB. When CE is switched

from one state to the next, there is a 50ns output

latchpropagationdelaybetweentheMSBsandLSBs

being present on the output pins.

CALIBRATION

The calibration procedure for the SP8121 consists of

adjusting the most negative input voltage (0V) to the

ideal output code for offset adjustment, and then

adjusting the most positive input voltage (5.0V) to its

ideal output code for gain adjustment.

Offset Adjustment

The offset adjustment must be completed first. Please

referto Figure1.Applyaninputvoltageof0.5LSBor

610

µV to any multiplexer input. Adjust the offset

potentiometersothattheoutputcodefluctuatesevenly

between 000…000 and 000…001. It is only

necessarytoobservethelowereightLSB’sduringthis

procedure.

Gain Adjustment

With the offset adjusted, the gain error can now be

trimmed to zero. (Please refer to Figure 2.) The ideal

input voltage corresponding to 1.5 LSB’s below the

nominal full scale input value, or +4.988V, is applied

to any multiplexer input. The gain potentiometer is

adjusted so that the output code alternates evenly

between 111…111 and 111…110. Again, only the

lower eight LSB’s need be observed during this

procedure. With the above adjustment made, the

converter is now calibrated.