Page 142 - Troubleshooting Analog Circuits

ADCs Can Be Tough and Temperamental I29

manufacturers don’t make much noise about noise. Ron Knapp of Maxim wrote a
nice explanation of this ADC noise measurement technique in EDN recently (Ref. 3).
I recommend his article on this subject.
Most ADC data sheets spell out that the only correct way to test or use an ADC is
with the analog signal’s ground, the digital supply’s ground, and the analog supply’s
ground tied together right at the ground pin of the ADC. If you don’t or can’t inter-
connect the grounds at the specified point, all bets are off.

n

CONVERT 0 MSB OUT
CONVERT
COMMAND * 0
COMMAND
ADC 0
I UNDER 0 LATCHES -
TEST (OPTIONAL) -
LSB+l lk
” -. . -,
A
,
2k
LSB OUT (OPTIONAL) 2k :;
LSB OUT
A
- 100
ANALOG
ANALOG
GROUND
GROUND
LOW-NOISE
REFERENCE-VOLTAGE SOURCE

I 0 0
20
e-1 VERTICAL HORIZONTAL
TRIANGULAR WAVE INPUT INPUT
GENERATOR
(1 TO 100 HZ AS -0
APPROPRIATE)

(b) (C) (d)
Figure 10.7. A reference source, a triangular-wave generator, and a scope are the major building blocks of
an ADC crossplot tester (a) that can reveal how much noise a converter adds to the signal it
is digitizing. In (b) the noise performance is ideal, whereas in (c) it is merely acceptable. In (d)
the noise performance is unacceptable.