Metered Measurements
The Optopatch includes a digital meter, with a selector switch to choose any of nine different parameters. The meter is autoranging, and one or other of two LED indicators illuminates to show whether the output is in volts or millivolts. The full-scale reading on the most sensitive range is nominally +/-199.9mV, but range switching occurs at about 160mV, to +/-1.999V full scale, and again at about 1.6V, to the top range of +/-19.99V full scale. When the meter reads the internal oscillator frequency, in which 1V represents 1KHz, an additional range-switching circuit comes into operation, so that oscillator frequencies in the 10-100KHz range are displayed as 10.0-100.0V.
Most of the other parameters that the meter can measure are described elsewhere in the manual, and the description of the front panel also provides a complete summary. However, the noise position is not described in detail elsewhere, so it is covered here.
This allows true rms measurement of the final output signal after gain and filtering so it can be used to measure any signal. The signals low frequency cut off is produced by a single pole filter at 100Hz and the high frequency cut off is set by the system output filter.
Signals are sent to an integrated circuit that performs an rms (root mean square) calculation of the signal levels. In retrospect, we rather wish we'd designed our own circuit to do this, as the single dedicated IC we use is by far the most expensive component in the entire Optopatch design, but at least it does the job (we hope...).
AC signal levels are normally quoted in rms units, as the equivalent power, i.e. when an rms voltage is multiplied by an rms current, is then the same as for DC signals of the same values. If an AC signal waveform is predictable, e.g. a sine wave, then it is possible to measure its average or peak value after rectification to give a DC voltage, and then to apply an appropriate conversion factor to give the rms value. However, by definition noise is random, so we cannot do that here, hence the use of a dedicated IC to do a proper rms calculation. Note that the squaring action also performs a rectification, so rms values are always positive.
The reason for providing this facility is to check the noise performance of the recording configuration. The noise measurement facility can be used in all operating modes, but it is primarily intended for true patch voltage clamp recordings. Under these conditions, 1V signal output represents 100pA current, so the x100 gain before the rms circuit means that 1V rms out represents 1pA current. Thus the rms noise level is shown directly in pA, or in fA when the meter is in its most sensitive range, i.e. when the mV LED is illuminated.
