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INDUCTIVE SPIKES AND PROBE SATURATION
Some care must be taken if using a differential probe to sense the logic output. While a
differential probe can be useful in reducing the possibility of interference from the high-
voltage pulse, it may have strict voltage limits. For example, the Tektronix P6246 is only
suitable for values of VCC2 up to +7V. A non-differential probe may be more suitable if
VCC2 > 7V.
A less obvious problem arises if parasitic inductances or capacitances in the test circuit
cause differential voltage spikes exceeding ±7V. The P6246 can saturate heavily under
those conditions, which can generate apparent glitch-like transients that are not due to
the DUT. Some experimentation may be required by the user in order to identify the
best probing arrangement.
Early versions of the AVRQ-5-B were provided with sample daughterboards P/N 267B
or earlier. (The current version is 267C). The earlier daughterboards used one pin to
connect GND2 to the main board ZIF socket. The inductance in the socket pin was
sufficient to generate a short voltage spike that could, under some circumstances,
overload the P6246 probe. For example, with a 267B daughterboard, a 1k pullup
resistor (R2), VCC = +5V, 300 pF between P1 and P2, and no DUT, the following
waveforms were obtained:
This shows an apparent 200 ns wide “glitch” on the logic output. In fact, the P6246 is
saturating from the sharp initial inductive voltage spikes, and is taking 200 ns to
recover.
To fix this issue, later daughterboards (267C and later) use four pins to connect GND2
to the main board ZIF socket. This reduces the initial inductive voltage spikes, and
prevents probe saturation:
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Summary of Contents for AVRQ-5-B
Page 52: ...Bottom side 52 ...
Page 57: ...PCB 299B FOR ADUM241E0BRWZ 57 ...
Page 82: ...PCB 158R2 LOW VOLTAGE POWER SUPPLY ...
Page 87: ...DUT WIRING ON STANDARD DAUGHTERBOARD PCB 267C ...
Page 88: ...DUT WIRING ON CUSTOMIZED TLP2366 DAUGHTERBOARD PCB 298B ...
Page 89: ...DUT WIRING ON CUSTOMIZED ADUM241E0BRWZ DAUGHTERBOARD PCB 299B ...
Page 91: ...PERFORMANCE CHECK SHEET 91 ...