Page 140 - Troubleshooting Analog Circuits

D/A Converters Are Generally Docile I27

Figure 10.5. An ordinary high-impedance THIS PROBE CAUSES TTL GATE
probe can cause TrL outputs TO OVERSHOOT AT HIGH-TO-LOW
TRANSITION
to appear to overshoot when
you look at them, but not
when you are not looking at \\
them. You can eliminate this I If
effect by making your own T-
1
'"tl SILVERED
PF
very-high-impedance probe, MICA
that presents only a I -pF
HIGH-SPEED
capacitive load. lOOk OSCILLOSCOPE

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Figure 10.6. The author irritates co-workers when he carries one of his large "choreographies" to the
photocopier and tries to figure out how to duplicate such a large drawing. In fact, some say
that he irritates co-workers most of the time.

supply voltages. In some cases, the DC rejection can be 80 or 100 dB, but high-fre-
quency noise on a supply can come through to the output virtually unattenuated. So
you must plan your system carefully. It might be a good idea, in a critical application,
to use a completely separate power-supply regulator for your precision DAC. At least
you should add plenty of good power-supply bypass capacitors right at the power-
supply pins--ceramic and tantalum capacitors.
Sometimes when you feed signals to a DAC without passing them through buffers,
the noise, ringing, and slow settling of the digital signals can get through to the
analog side and show up on the DAC output. Nobody has a spec for rejection of the
noise on DAC bit lines in either the HIGH or LOW state. Maybe vendors should