【正文】 stems, for historical reasons, the term DMA refers to the use of the DMA controller built in to the motherboard.Data Transfer: OutputThe discussion so far has concentrated on data transfer for acquired data. If the digitizer is used for synchronous stimulation or control, the same data transfer problem occurs as for acquiring data. In fact, the total data rate doubles. Consider, for example, a stimulus/response measurement on one channel with a 100kHz sampling rate. Acquired data is received by the puter at 100kHz. Simultaneously, the stimulus waveform must be delivered by the puter to the digitizer at 100kHz. The full data rate Axon Instruments and Instrutech digitizers have symmetric handling of inputs and outputs. The output buffers are the same size as the input buffers, and the same data transfer technique is used.Measurement AccuracyThe following sections discuss the issues that influence the accuracy of dynamic measurements.CrosstalkMost digitizers record from multiple analog input channels, with 8 or 16 input channels being monly supported. An important specification is the crosstalk between input channels, that is, the amount of input signal from one channel that appears on another channel.Crosstalk is a problem because many digitizers use a single analog to digital converter, and a switch called a multiplexer to select between input channels.The multiplexer itself is a source of crosstalk. Even when a switch is open, capacitive coupling between the input of the switch and the output of the multiplexer produces a frequencydependent crosstalk. Highfrequency input signals are coupled to the multiplexer output even when they are not selected.To measure such crosstalk, ground an analog input and sample from it. Meanwhile, connect a highfrequency signal to other input channels. Notice the amplitude of the highfrequency signal that appears on the grounded input. This is the crosstalk. Vary the input frequency and notice the change in the amount of crosstalk.Crosstalk may not be significant when a digitizer is used for patchclamp data acquisition. Typically one analog input is used for the ion channel signal, while other analog inputs are used to measure very lowfrequency signals. The lowfrequency signals do not couple significantly to the ion channel signal. The ion channel signal does couple into the lowfrequency channels, but this can generally be eliminated by averaging many input samples on those channels.If you measure on several channels containing high frequency data, characterize the crosstalk of your data acquisition system before you do so. Otherwise you may find yourself measuring correlations in input data due to your digitizer instead of the system being measured.This problem will bee less significant with time, as the cost of A/D converters drops. Digitizer manufacturers can afford to place one A/D converter for each input channel, avoiding the use of a multiplexer.Settling TimeThe settling time of the A/D converter input may limit the rate of multichannel sampling.The input amplifiers on many A/D converters cannot follow very high frequency input signals. When the multiplexer switches channels, this appears as a sudden jump in signal level to the input of the A/D converter. At low sampling rates, the A/D input will have considerable time to settle before converting the next sample. At high sampling rates, the input may not have time to settle, and the input signal on one channel affects the value measured on the next.To see this effect, ground all inputs of a digitizer except one. Connect this input to a variable DC level. Sample at a high rate on multiple channels. Notice if changing the input level on one channel causes the value measured on one of the grounded channels to change.Frequently, digitizers achieve full bandwidth only when the multiplexer is not being used, and the digitizer is sampling from only a single input channel.The Axon Instruments Digidata 1200A/B and the Instrutech Corporation ITC16 both use a single A/D converter and a multiplexer. The Instrutech Corporation ITC18 uses a separate A/D converter per input channel. While this raises the cost of the device, it essentially eliminates crosstalk.GroundingThe digitizer is electrically part of your instrumentation system. This can cause problems if you do not consider the digitizer when planning the grounding of your instrumentation.If your digitizer is used only for acquisition, you can take advantage of differential analog inputs to avoid connecting your digitizer directly to your measurement ground through signal cables. However, if you use the analog outputs of your digitizer this may not be possible, since analog outputs are rarely differential.Analog outputs are particularly a problem if the digitizer ground is the same as the puter ground. Computer ground lines usually transmit highfrequency switching noise. The noise can be coupled through the mon ground into your measurement system. This is a mon failing of lowcost digitizer boards.The Instrutech ITC16 and ITC18 use optical isolation in the digital control path of the digitizer. This pletely isolates the measurement system from the puter ground.Input ImpedanceThe FETbased input amplifiers used in modern digitizers have a very high input impedance. If inputs are left unconnected, they can pick up unwanted signals and couple them into the digitizer.The Axon Instruments Digidata 1200A/B and the Instrutech ITC16 have very high impedance analog inputs. For best results, unused inputs on these devices should be grounded.The Instrutech ITC18 has bleed resistors connected internally between the analog inputs and ground to reduce pickup of stray signals. Grounding of unused analog inputs is less critical with this device.PhaseIf you are sampling from multiple input channels, you may be interested in the phase relationship between the inputs.Digitizers that use a single multiple