【正文】 10V, matching the range of mon instrumentation quality digitizers.DigitizerA 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 puter.Sampling TheoremThe 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.ControlMost of this discussion is about digitizing analog signals for a puter. In many cases, a puter also produces analog control signals. For example, in patchclamp experiments involving voltagegated ion channels, the puter 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 SignalsMany signal sources consist of a sensor and an amplifier. The amplifier converts the output of the sensor into the signal to be digitized.PreamplifierMany instrumentation systems are built with a preamplifier located as close to the sensor as possible. A separate amplifier converts the preamplifier output to a highlevel 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 solutionfilled pipette, the preamplifier is the head stage, and the amplifier is the patchclamp amplifier itself.Signal ConditioningMany 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 currenttovoltage 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 plex. An input signal from a nonlinear sensor may be converted to a voltage that is linear in the quantity being measured, pensation may be made for secondorder effects such as temperature, or an indirect effect such as a frequency shift may be converted to a voltage.Integrated DigitizerAs the cost of A/D converters declines, the digitizing function can be moved into the amplifier. For example, the HEKA elektronik EPC9 patchclamp amplifier contains a built in digitizing unit (an Instrutech ITC16).Integrating a digitizer into an amplifier can substantially reduce total noise in the digitized signal, since the analog signal is not carried over a cable from the amplifier to an external digitizer. Be careful of instrument specifications when paring an analog amplifier to one with a builtin digitizer. Including the digital electronics in the amplifier housing may increase noise, and the digitizer itself may add noise to the signal. However, the total noise in the digitized signal may be much less than if an external digitizer is used. Compare an amplifier with an integrated digitizer to the bination of an analog amplifier and an external digitizer.A major advantage of integrating a digitizer into an amplifier is that the amplifier designer can easily include features for puter control. A data acquisition program connected to such an amplifier can then offer an integrated user interface, simplifying operation. In addition, the acquisition program can record all amplifier settings, simplifying data analysis.From Signals to SamplesA digitizer consists of an A/D (analog to digital) converter that samples an analog input signal and converts it to a sequence of digital values.AliasingThe sampling theorem states that, in order to be able to reconstruct a signal, the sampling rate must be at least twice the signal bandwidth. What happens if a signal contains ponents at a frequency higher than half the sampling frequency? The frequency ponents above half the sampling rate appear at a lower frequency in the sampled data.The apparent frequency of a sampled signal is the actual frequency modulo half of the sampling rate. For example, if a 26kHz signal is sampled at 50kHz, it appears to be a 1kHz signal in the sampled data. This effect is called aliasing.AntiAliasing FilterIf a signal to be digitized has ponents at frequencies greater than the half the sampling frequency, an anti aliasing filter is required to reduce the signal band width. The antialiasing filter must cut off signal ponents above one half the sampling rate.Most signal sources are inherently bandlimited, so in practice, antialiasing filters are often not required. However, some signal sources produce broadband noise that must be removed by an antialiasing filter.For example, patchclamp amplifiers have builtin anti aliasing filters. The pipette used for patchclamp recordi