【正文】 gnal. 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 recording inherently filters signals above a low frequency in the range of 1kHz. The good high frequency response of a patch clamp amplifier is achieved only by boosting the high frequency ponent of the signal to pensate for the frequency response of the pipette. This can produce significant highfrequency noise. A patchclamp amplifier provides a filter to eliminate this noise.Integrating ConvertersThe discussion of aliasing assumes instantaneous sampling. The output value produced by the A/D is represents the instantaneous analog signal amplitude. Such sampling A/D converters are the most mon for use in instrumentation.Some A/D converters employ an integrating conversion technique. The output value produced by such a digitizer represents the integral of the analog signal amplitude over the sampling interval. Such converters eliminate aliasing. They can be viewed as containing a builtin antialiasing filter.Integrating converters are rarely used in highspeed control applications. The most mon techniques for implementing highspeed integrating converters result in a delay of many sample intervals between an analog sample and the corresponding digitizer output value. This delay can introduce considerable phase shift at high frequencies in closedloop response if the digitizer is used in a control system.ResolutionTypically a digitizer provides the puter with fixed length binary numbers. For example, the Axon Instruments Digidata 1200A produces 12bit numbers, while the Instrutech Corporation ITC16 produces 16bit numbers. The length of each value is called the resolution of the device, measured in bits.The resolution can be translated to an absolute input level. Most digitizers measure swings of up to approximately 10V from zero, for a total range of 20V. A 12bit value has a resolution of 1 part in 4096, so the resolution of a 12bit digitizer is 20V divided by 4096, or approximately 5mV. This is expressed by saying that a change of one count (or one least significant bit, or LSB) represents 5mV.Since analog instruments rarely have an accuracy significantly exceeding %, it might seem that 10 or 11 bit resolution would be suf