User`s manual

XP8500 and Exp-A/D12102 s Technical Circuit Details
Exp-A/D12
Input Stability
Grounded input signals were used to calculate the standard deviations of
A/D converter channels. Tests of 1000 samples per channel were per-
formed on typical boards in both the unipolar and bipolar modes. Under
these conditions, the standard deviation of the lower gain channels (gains
of 1, 6, 22, and 42) in bipolar mode was typically 0.10.4 (out of 2047).
The standard deviation for the higher gain channels (gains of 102 and 202)
was typically 0.60.8. All channels consistently had a standard deviation
under 1.0, although the standard deviation for the 202X channel occasion-
ally ranged up to 0.95.
Predictably, the numbers in unipolar mode were about double those
acquired in bipolar mode. The lower gain channels consistently had a
standard deviation under 1.0, typically 0.20.9. The standard deviation for
the higher gain channels ranged from 1.2 to 1.9.
Effects of the OP6300 on Stability
Z-Worlds OP6300 operator interfaces can significantly affect the stability
of the high-gain channels on the Exp-A/D12. This is because of the VCC
noise generated by the switching negative power supply used for the
graphic LCD. With an OP6300 attached to the Exp-A/D12, the standard
deviation rose from 0.6 to 3.2 for 1000 samples. When capacitor C1 on
the OP6300 graphic board was changed from 10 µF at 16 V to 100 µF at
16 V, the standard deviation fell to approximately 0.8.
The interference can also be eliminated by connecting the OP6300
between the Exp-A/D12 and the controller rather than at the end of the
PLCBus chain. This requires special cabling.
A character-display board can be attached to the Exp-A/D12 with no
adverse effect on the stability of inputs.
Multiplexer Settling Time
When switching multiplexers, be sure to allow the circuitry to settle before
attempting to read the A/D output. Capacitors C10C14 (330 pF) attached
to the feedback on gain amplifiers U2D and U3AU3D are the greatest
contributors of settling delay.
The time constant formed by this RC combination increases as the gain
increases. For the worst case, the 202X channel,
RC = 330 pF × 100 kΩ = 33 µs.
The time constant on this channel was measured by scope to be 50 µs.
Since it takes 9 time constants for the signal to settle to within ±1 LSB for
the LTC1294, the maximum settling time is 450 µs.