Analog to Digital (A/D) Converter

TM 11-6625-444-14-2

b. Ratio Differential Amplifier. A dc reference

o f the ratio differential amplifier, A9-U1, is

voltage of either plus or minus polarity is con-

always a positive voltage. The polarity sign of

netted to the RATlO (EXTERNAL REFERENCE)

the displayed percent ratio will always be the

rear input terminals. Polarity detector A9-U54A

algebraic equivalent of the input signal divided

will reverse the polarity of a negative reference

by the reference signal. Refer to table 5-2.

v o l t a g e by actuating relay A9-K16 so the output

Table 5-2. Ratio Polarity

External Reference

Displayed

Input Signal

Polarity

A9-U1 has unity gain for reference voltages up

the auto ranging circuits. See table 5-3 for a

to 10.5 V but changes automatically to a gain of

listing of ratio ranging possibilities.

0.1 for reference voltages between 10.5 and

100 V; thus the maximum full scale output of the

5-5. Analog to Digital (A/D) Converter

ratio differential amplifier is +10.5 volts.

a. The resultant analog dc signal from the A/D

buffer, AI-U 104, is applied to the analog to dig-

ital (A/D) converter. Here it is compared to a

c. The ratio voltage is then applied through

+10 V reference if the input signal is positive or

active filter A9-U100 (similar to that described in

to a -10 V reference if the input signal is nega-

p a r a g r a p h 5-3. c.) to the ratio output buffer

tive (dc voltage mode only; its polarity auto-

amplifier, A9-U450. A9-R452, the RATIO FULL

m a t i c a l l y detected and displayed by the A/D

RANGE control, permits slight gain adjustments

converter. The analog signal is converted to a

(less than/more than unity) of this amplifier so

digital signal that is equivalent to 199,000 counts

that when the reference voltage equals the input

from the 12 MHz clock for a full scale reading,

voltage; the displayed ratio reading can be rror-

each time a sample reading is taken by the digi-

malized to 100.0007.

A voltage measurement that is

tal counter.

one-tenth the full scale range results in a count

d. Ratio Autoranging Operation. The ratio

of 20,000 when a sample reading is taken.

amplifier voltage is then compared with the dc

input voltage in the ratio up/down detectors,

b. The sampled pulses are applied to digital

A1-U402 and U401 respectively. The output of

counters (A2-U354 through U358) and then sent

the detectors, is an "up range command" for ra-

to the 5½ digit display (A3) where they are pre-

tio counts higher than 180,000 or a "down range

sented as a voltage or ratio reading with polarity

command" for ratio counts lower than 14,000

which is then applied to the up/down counter,

automatically indicated. The count is also con-

A2-U51. This determines how many pulses from

verted to BCD and made available for external

the auto range clock, A2-U60, are processed by

processing at the rear panel.

5-3