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Data Acquisition: An Introduction

Data Acquisition:An Introduction

Bruxton Corporation

This is an informal introduction digital data acquisition hardware. It is prima-rily 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/ con!erter deli!ers a sequence of !alues represent-ing ananalog signal to an acquisition program. In practice, selecting and properly using data acquisition hardware is morecomplex. This document pro!ides an informal intro-duction to the topic.

ContentsBac"ground #$rom %ensors to %ignals &$rom %ignals to %amples &$rom %amples to Computer '(easurement Accuracy )

(any of the examples are ta"en from patch-clamp recording. This technique requires accurate acquisition of low-le!elsignals *picoamperes+ with andwidth in the audio range *up to # " +.

BackgroundA data acqui-

Amplier

Digitizersition system

con-!erts a signal Sensor +3.250deri!ed from a +3.100+2.500sensor into a +1.745sequence of digi- +0.985

tal !alues. The sensor is connected to an amplifier, which con!erts the signal into a potential. The amplifier is in turn

connected to a digiti er, which contains an A/ con!erter. The digiti er produces a sequence of !alues representing thesignal.

Signal Source

The source of most signals to e digiti ed is a sensor, connected to an amplifier with appropriate signal condi-tioning. Theamplifier deli!ers an electrical signal. This sig-nal is then digiti ed using an A/ con!erter.

$or patch-clamp recording, the sensors are solution-filled pipettes. The pipette is connected to a patch-clamp amplifier that con!erts the !oltage at the pipette or the cur-rent through the pipette to a high-le!el signal. By con!en-tion, the full-

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scale output range of a patch-clamp amplifier is 0# 1, matching the range of common instrumentation-quality digiti ers.

Digitizer

A digiti er con!erts one or more channels of analog signal to a sequence of corresponding digital !alues. The heart of adigiti er is an A/ con!erter, a de!ice that sam-ples an analog signal and con!erts the sample to a digital !alue.

$or example, for recording from a single ion channel, the digiti er might determine the output of the patch clamp

amplifier once e!ery ) s and pro!ide the resulting !alue to the computer.

Sampling Theorem

The pur- pose of data acquisition isto analy e an analog signal in digitalform. $or this to e possi le, the sequence of !alues pro-duced y a digiti er must represent the original analog sig-nal. Thesampling theorem states that this is the case.

23. 4.#4 5age # of 6Data Acquisition: An Introduction

The sampling theorem states that an analog signal can e reconstructed from a sequence of samples ta"en at a uni-forminter!al, as long as the sampling frequency is no less than dou le the signal andwidth. $or example, assume a signalcontains frequencies from C * + to # " . This signal must e sampled at a rate of at least & " to e reconstructed

properly.As a practical matter, the sampling rate should e se!-eral times the minimum sampling rate for the highest fre-quency of

interest. $or example, to resol!e a # " signal, a minimum sampling rate of & " is required, ut a sam-pling rate of ) " or more should e used in practice.

Control(ost of this discussion is a out digiti ing analog sig-nals for a computer. In many cases, a computer also pro-duces analog

control signals. $or example, in patch-clamp experiments in!ol!ing !oltage-gated ion channels, the com-puter is frequently usedto produce an electrical stimulus to acti!ate the channels. These control signals are produced using a /A *digital to analog+con!erter.

rom Sensors to Signals

(any signal sources consist of a sensor and an ampli-fier. The amplifier con!erts the output of the sensor into the signalto e digiti ed.

!reampli"ier

(any instrumentation systems are uilt with a pream-plifier located as close to the sensor as possi le. A separate amplifier con!erts the preamplifier output to a high-le!el signal. 5lacing the preamplifier close to the sensor reduces noise, y allowing thesignal to e amplified efore eing sent o!er a ca le. %ince physical space near the sensor is limited, the preamplifier is as small as

possi le, with the ul" of the electronics eing located in the amplifier.$or Headstage Amplier

example, in a(Preamplier)

patch clampMicroelectrodesetup, the

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sensor

is a

solution-filled pipette, the preamplifier is the head stage, and the amplifier is the patch-clamp amplifier itself.From Sensors to Signals

Signal Conditioning

(any sensors deli!er signals that must e transformed efore they can e digiti ed. $or example, a microelectrode pipette may eused to measure current, while the digiti er measures potential *!oltage+. The patch clamp amplifier pro!ides a current-to-!oltageamplification, usually mea-sured in m1 of output per pA of input. This transformation of the sensor signal is called signal conditioning.

%ignal conditioning may e more complex. An input signal from a non-linear sensor may e con!erted to a !olt-age thatis linear in the quantity eing measured, compen-sation may e made for second-order effects such as temperature, or anindirect effect such as a frequency shift may e con!erted to a !oltage.

Integrated Digitizer

As the cost of A/ con!erters declines, the digit ing function can e mo!ed into the amplifier. $or example, the 78Aele"troni" 75C-2 patch-clamp amplifier contains a uilt in digiti ing unit *an Instrutech ITC-#9+.

Integrating a digiti er into an amplifier can su stan-tially reduce total noise in the digiti ed signal, since the analog signal isnot carried o!er a ca le from the amplifier to an external digiti er. Be careful of instrument specifica-tions when comparing ananalog amplifier to one with a uilt-in digiti er. Including the digital electronics in the amplifier housing may increase noise, andthe digiti er itself may add noise to the signal. owe!er, the total noise in the digiti ed signal may e much less than if an externaldigiti er is used. Compare an amplifier with an integrated digiti er to the com ination of an analog amplifier and an externaldigiti er.

A ma:or ad!antage of integrating a digiti er into an amplifier is that the amplifier designer can easily include featuresfor computer control. A data acquisition program connected to such an amplifier can then offer an integrated user interface,simplifying operation. In addition, the acqui-sition program can record all amplifier settings, simplifying data analysis.

rom Signals to Samples

A digiti er consists of an A/ *analog to digital+ con-!erter that samples an analog input signal and con!erts it to asequence of digital !alues.

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Aliasing

The sampling theorem states that, in order to e a le to reconstruct a signal, the sampling rate must e at least twice thesignal andwidth. ;hat happens if a signal con-tains components at a frequency higher than half the sam-pling frequency