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NIDisk User Guide

the input signal is V(t) = A sin 2πft, then voltage change is dV/dt = 2πfA cos 2πft, whose

maximum = 2

πfA Volts/sec. So instantaneous voltage change depends on the product of

input amplitude (A) and frequency (f) and will be greatest for a full-scale input signal of

maximum (Nyquist) frequency.

Under these worst-case conditions, and for input gains between 1 and 10, NI-specified

settling time is 1 usec to 12 bit accuracy and 1.5 usec to 16 bit accuracy. Thus at unity gain

(10 Volt range), a 10-Volt input signal at Nyquist frequency requires 1.5 usec to settle to 16-

bit accuracy. Smaller signals require less time to settle; for example, a 0.5-Volt input signal

acquired at unity gain has only 12 bits to settle and requires closer to 1 usec for a 50% greater

BW. NI-specified maximum "small-signal" BW is 1.7 MHz, a settling time of 0.6 usec.

Acquisition Crosstalk

Settling affects multi-channel operation via the voltage difference between successive

channels. As total channel BW approaches the board's BW capability, the input amplifier

does not have time to settle between successive channel samples, and carries the value from

one channel into the next. This artifact is called crosstalk and is defined as the ratio of

amplitude induced in one channel by a signal in another. Crosstalk is measured by shorting

the target channel input, applying a full-scale signal to the previous channel, and measuring

leakage into the target channel. NI specifies worst-case crosstalk as -75 dB, but measured

performance exceeds that.

The following table summarizes measured crosstalk vs. total bandwidth = (sample rate) *

(no. channels), and can be used to limit board operation according to desired accuracy.

Measurements were made with 8-channel operation.

Crosstalk level

Bandwidth

-87 dB 1.00 MHz

-65 dB 1.33 MHz

-47 dB 1.43 MHz

-20 dB 1.54 MHz

For example, if the board is operated at 1.33 MHz (e.g., 8 channels at 167 kHz sample rate),

crosstalk will be -65 dB, limiting accuracy to about 11 bits, which is acceptable for many

biological applications, while at 1.43 MHz (8 channels at 178 kHz), crosstalk is -47 dB.

13. Using NIDisk and SIGNAL

NIDISK can be used in conjunction with the SIGNAL

sound analysis program to record

extended acoustic data sequences, then locate and analyze the sound events within them.

This section describes the overall approach, basic tools, and important information sources.

The two programs work together as follows:

• NIDisk provides long-duration high-frequency direct-to-disk recording, producing long

continuous sound files in the SIGNAL or Wave sound file format.

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Engineering Design NIDisk Guía de usuario Pagina 32