500-003111-000
3-17
3.9.2
Loopback Testing of Inputs and Outputs
By routing, the analog outputs through the analog input multiplexers
(see previous section), all active components are exercised in a "loopback"
arrangement. The controlling processor can perform a loopback test in either
the on-line or off-line mode by sending a voltage-level code to a specific output
channel, and by then verifying that the ADC produces the same code after
sampling the signal. This technique is described in detail in Section 4.
3.9.3
Self-Test Standards
The on-board CAL TEST VOLTAGE REFERENCE is
jumper-configurable to one of the following levels as nominally ±2.5 VDC,
±5.0 VDC, or ±10 VDC (actual voltages are slightly less than these in order to
ensure in-range conversions during self-test and autocalibration). To provide
both positive and negative references for testing and calibrating bipolar ranges,
the CAL TEST voltage is routed to both the HIGH (noninverting) and LOW
(inverting) inputs of the self-test multiplexer.
3.9.4 Gain
Autocalibration
As shown in Figure 3.9.4-1, gain autocalibration is implemented by
adjusting the reference input of the ADC. The output of the 12-bit autogain DAC
is offset and attenuated to produce a gain adjustment range of ±1.2 percent
which corresponds to an adjustment resolution of 0.0006 percent per DAC LSB.
Control data for the gain DAC is written as 12 bits, right-justified, into a 16-bit
WRITE ONLY register located at relative address 0A (HEX) in the VMIVME-
3111 assigned address space.
3.9.5 Zero
Autocalibration
In any high-gain, direct coupled, signal path, zero offset errors at the
input are multiplied by the path gain and can, therefore, produce large output
errors. For this reason, zero autocalibration in the VMIVME-3111 is performed
at the input to the Programmable Gain Amplifier (PGA). As shown in Figure
3.9.4-1, the PGA accepts the output of the 12-bit autozero DAC. Autozero
adjustment
range is ±10 mV, which correspond to an adjustment resolution of 4.7 µV per
DAC LSB.
3.10
BUILT-IN POWER CONVERTER
Electrical power for the VMIVME-3111 analog networks is supplied by
the DC-to-DC Converter shown in Figure 3.10-1. The converter transforms 5 V
logic power into regulated and isolated ±15 VDC power with a load capacity of
approximately 190 mA on each 15 V bus.
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