294

latch the 8 MSB’s and 4 LSB’s of output data on

the 8-bit wide output data bus.

The conversion cycle is started when R/C is

brought low and must be held low for a mini-

mum of 50ns. The R/C signal will also cause the

output latches to be in a tri-state mode when low.

Approximately 200ns after R/C is low, STA-

TUS will change from low to high. This output

signal will stay high while the SP8480 is per-

forming a conversion. Valid data will be latched

to 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 5. To read data from the

SP8480, the R/C and A

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. A

MSBs will be on pins 26 through 19, with pin 26

being the MSB. The remaining 4 LSBs will be on

pins 23 through 26 with pin 23 being the LSB.

When A

there is a 50ns output latch propagation delay

between the MSBs and LSBs being present on

the output pins.

CALIBRATION

The calibration procedure for the SP8480 con-

sists of adjusting the most negative input voltage

(0V) to the ideal output code for offset adjust-

ment, and then adjusting the most positive input

voltage (5.0V) to its ideal output code for gain

adjustment.

SP8480

GAIN ADJUST

125K

Ω

+15V

10K

Ω

±0.3% Trim Range

5

19K

Ω

Center pot

for zero

correction

Figure 2. Gain Adjust

Offset Adjustment

The offset adjustment must be completed first.

Please refer to Figure 1. Apply an input voltage

of 0.5LSB or 610

µV to any multiplexer input.

Adjust the offset potentiometer so that the output

code fluctuates evenly between 000…000 and

000…001. It is only necessary to observe the

lower eight LSB’s during this procedure.

Gain Adjustment

With the offset adjusted, the gain error can now

be trimmed to zero (see 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 poten-

tiometer 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.

SP8480

OFFSET ADJUST

100K

Ω

+15V

5K

Ω

–1.5mV to +3mV

4

Figure 1. Offset Adjust