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日期：2013年6月4日
参考文献
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附录A 英文文献原文
Data Acquisition: An Introduction
Bruxton Corporation
This is an informal introduction digital data acquisition hardware. It is primarily directed towards assisting in the selection of appropriate hardware for recording with the Acquire program. Overview
In principle, data acquisition hardware is quite simple. An A/D converter delivers a sequence of values representing an analog signal to an acquisition program. In practice, selecting and properly using data acquisition hardware is more complex. This document provides an informal introduction to the topic.
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Many of the examples are taken from patch-clamp recording. This technique requires
accurate acquisition of low-level signals (picoamperes) with bandwidth in the audio range (up to 10kHz).
Background
A data acquisition system converts a
AmplifierDigitizersignal derived from a sensor into a sequence sensor+3.250of digital values. The sensor is connected to
+3.100an amplifier, which converts the signal into a +2.500+1.745potential. The amplifier is in turn connected
+0.985to a digitizer, which contains an A/D
converter. The digitizer produces a sequence of values representing the signal. Signal Source
The source of most signals to be digitized is a sensor, connected to an amplifier with appropriate signal conditioning. The amplifier delivers an electrical signal. This signal is then digitized using an A/D converter.
For patch-clamp recording, the sensors are solution filled pipettes. The pipette is connected to a patch-clamp amplifier that converts the voltage at the pipette or the current through the pipette to a high-level signal. By convention, the full-scale output range of a patch-clamp amplifier is ±10V, matching the range of common instrumentation quality digitizers. Digitizer
A digitizer converts one or more channels of analog signal to a sequence of corresponding digital values. The heart of a digitizer is an A/D converter, a device that samples an analog signal and converts the sample to a digital value.
For example, for recording from a single ion channel, the digitizer might determine the output of the patch clamp amplifier once every 50ms and provide the resulting value to the computer.
Sampling Theorem
The purpose of data acquisition is to analyze an analog signal in digital form. For this to be possible, the sequence of values produced by a digitizer must represent the original analog signal.
The sampling theorem states that this is the case. The sampling theorem states that an analog signal can be reconstructed from a sequence of samples taken at a uniform interval, as long as the sampling frequency is no less than double the signal bandwidth. For example, assume a signal contains frequencies from DC (0Hz) to 10kHz. This signal must be sampled at a rate of at least 20kHz to be reconstructed properly.
As a practical matter, the sampling rate should be several times the minimum sampling rate for the highest frequency of interest. For example, to resolve a 10kHz signal, a minimum sampling rate of 20kHz is required, but a sampling rate of 50kHz or more should be used in practice. Control
Most of this discussion is about digitizing analog signals for a computer. In many cases, a computer also produces analog control signals. For example, in patch-clamp experiments involving voltage-gated ion channels, the computer is frequently used to produce an electrical stimulus to activate the channels. These control signals are produced using a D/A (digital to analog) converter.
From Sensors to Signals
Many signal sources consist of a sensor and an amplifier. The amplifier converts the output of the sensor into the signal to be digitized. Preamplifier
Many instrumentation systems are built with a preamplifier located HeadstageAmplifieras close to the sensor as possible. A separate amplifier converts the
Microelectrodepreamplifier output to a high-level
signal. Placing the preamplifier close to the sensor reduces noise, by allowing the signal to be amplified before being sent over a cable. Since physical space near the sensor is limited, the preamplifier is as small as possible, with the bulk of the electronics being located in the amplifier.
For example, in a patch clamp setup, the sensor is a solution-filled pipette, the preamplifier is the head stage, and the amplifier is the patch-clamp amplifier itself. Signal Conditioning
Many sensors deliver signals that must be transformed before they can be digitized. For example, a microelectrode pipette may be used to measure current, while the digitizer measures potential (voltage). The patch clamp amplifier provides a current-to-voltage amplification, usually measured in mV of output per pA of input. This transformation of the sensor signal is called signal conditioning.
Signal conditioning may be more complex. An input signal from a non-linear sensor may be converted to a voltage that is linear in the quantity being measured, compensation may be made for second-order effects such as temperature, or an indirect effect such as a frequency shift may be