Measurement-computing Data Acquisition Systems rev.10.4 Manuel d'utilisateur télécharger pdf (Page 33)

Zero Compensation

Zero compensation removes offset errors

while a program is running. This is useful

in systems where the offset of a channel

may change due to temperature changes,

long-term drift, or hardware calibration

changes. Reading a shorted channel on the

same card at the same gain as the desired

channel removes the offset at run-time.

Note: Zero compensation is not available

for all expansion cards. The DBK19 has

channel 1 permanently shorted for zero

compensation; other cards require a

channel to be shorted manually.

The zero-compensation functions require a

shorted channel and a number of other

channels to be sampled from the same card

at the same gain as the shorted channel.

These functions will work with cards

(such as the DBK12, DBK13, and DBK19)

that have one analog path from the input to

the A/D converter. Other cards do not

support the zero compensation functions

because they have offset errors unique to

each channel. The DBK19 is designed

with channel 1 already shorted for

performing zero compensation.

The

daqZeroSetup function configures the location of the shorted channel and the channels to be zeroed

within a scan, the size of the scan, and the number of readings to zero compensate. (This function does not

do the conversion.) A non-zero return value indicates an invalid parameter error.

In this example, several Daq device channels will be read using various gains and zero-compensated to

remove any offset errors. This example assumes that channel 0 of the Daq device has been manually

shorted. Expansion cards could perform the same type of zero compensation as this example by shorting a

channel on the expansion card and specifying card channel numbers. First list the configuration:

Channel Channel Type

0 Shorted Channel

1 Voltage1 @ X1 gain

2 Voltage2 @ X2 gain

3 Voltage3 @ X2 gain

4 Voltage4 @ X2 gain

Now specify the scan, the sequence of channel numbers, and gains that are to be gathered as one burst of

readings. In this example, we will first read the shorted channel at each gain that we plan on using, in this

case ×1 and ×2. All the channels at each gain will be read together to make the actual zero compensation

easier.

Scan

Position

Channel Type Channel Gain

Code

0 Shorted Channel @ X1 0 DgainX1

1 Shorted Channel @ X2 0 DgainX2

2 Voltage1 @ X1 gain 1 DgainX1

3 Voltage2 @ X2 gain 2 DgainX2

4 Voltage3 @ X2 gain 3 DgainX2

5 Voltage4 @ X2 gain 4 DgainX2

Programmer’s Manual 988594 API Programming, General Models 2-23

1 2 ... 28 29 30 31 32 33 34 35 36 37 38 ... 365 366

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Measurement-computing Data Acquisition Systems rev.10.4 Manuel d'utilisateur Page 33

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