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MM74C905N

12-Bit Successive Approximation Register

Fairchild Semiconductor 

Switching Time Waveforms

USER NOTES FOR A/D CONVERSION

The register can be used with either current switches that

require a low voltage level to turn the switch ON or current

switches that require a high voltage level to turn the switch

ON. If current switches are used which turn ON with a low

logic level, the resulting digit output from the register is

active low. That is, a logic “1” is represented as a low volt-

age level. If current switches are used which turn ON with a

high logic level, the resulting digit output is active high. A

logic “1” is represented as a high voltage level.

For a maximum error of ±½ LSB, the comparator must be

biased. If current switches that require a high voltage level

to turn ON are used, the comparator should be biased +½

LSB and if the current switches require a low logic level to

turn ON, then the comparator must be biased −½ LSB.

The register can be used to perform 2's complement con-

version by offsetting the comparator one half full range +½

LSB and using the complement of the MSB Q11 as the sign

bit.

If the register is truncated and operated in the continuous

conversion mode, a lock-up condition may occur on power-

ON. This situation can be overcome by making the START

input the “OR” function of CC and the appropriate register

output.

The register, by suitable selection of register ladder net-

work, can be used to perform either binary or BCD conver-

sion.

The register outputs can drive the 10 bits or less with 50k/

100k R/2R ladder network directly for VCC = 10V or higher.

In order to drive the 12-bit 50k/100k ladder network and

have the ±½ LSB resolution, the MM74C902 or

MM74C904 is used as buffers, three buffers for MSB

(Q11), two buffers for Q10, and one buffer for Q9.

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