National Instruments DAQCard-1200

Chapter 4 Theory of Operation

can scan any number of channels from eight to two. Notice that the same

gain setting is used for all channels in the scan sequence.

The programmable gain amplifier applies gain to the input signal, allowing

an analog input signal to be amplified before being sampled and converted,

thus increasing measurement resolution and accuracy. The instrumentation

amplifier gain is software selectable. The DAQCard-1200 provides gains of

1, 2, 5, 10, 20, 50, and 100.

The dither circuitry, when enabled, adds approximately 0.5 LSB rms of

white Gaussian noise to the signal to be converted to the ADC. This

addition is useful for applications involving averaging to increase the

resolution of the DAQCard-1200 to more than 12 bits, as in calibration. In

such applications, which are often lower frequency in nature, noise

modulation is decreased and differential linearity is improved by the

addition of the dither. For high-speed 12-bit applications not involving

averaging, dither should be disabled because it only adds noise.

When taking DC measurements, such as when calibrating the board, enable

dither and average about 1,000 points to take a single reading. This process

removes the effects of 12-bit quantization and reduces measurement noise,

resulting in improved resolution. Dither, or additive white noise, has the

effect of forcing quantization noise to become a zero-mean random variable

rather than a deterministic function of input. For more information on the

effects of dither, see “Dither in Digital Audio,” Journal of the Audio

Engineering Society.

The DAQCard-1200 uses a 12-bit successive-approximation ADC. The

12-bit resolution of the converter allows the converter to resolve its input

range into 4,096 different steps. The ADC has an input range of ±5V and

0 to 10 V.

When an A/D conversion is complete, the ADC clocks the result into the

A/D FIFO. This FIFO serves as a buffer to the ADC. The A/D FIFO can

collect up to 1,024 A/D conversion values before any information is lost,

thus allowing software some extra time to catch up with the hardware. If

you store more than 1,024 samples in the A/D FIFO before reading from

the A/D FIFO, an error condition called A/D FIFO overflow occurs and you

lose A/D conversion information.

The output from the ADC can be interpreted as either straight binary or

two’s complement, depending on which input mode you select (unipolar or

bipolar). In unipolar mode, the data from the ADC is interpreted as a

12-bitstraig ht binary number with a range of 0 to +4,095. In bipolar mode,

the data from the ADC is interpreted as a 12-bit two’s complement number