Rear Panel
BNC Sokets and Associated Switches
The following BNC inputs and outputs are available. Please refer to the appropriate section of the manual for more detailed information than that given here.
VBath
The bath potential input (unity gain). This and other inputs can be left floating if not used, as all inputs have a relatively low input impedance of 10K ohms.
I=0 Current (or FreqTest) In
This is normally a current clamp command input, but it is active ONLY in I=0 mode, i.e. when the Optopatch is otherwise acting as a high-impedance voltage follower. The full-scale current sensitivity is the same as the headstage current output, i.e. either 1nA or 100nA for a 10V input with the standard headstage. Alternatively, this input can be switched to drive an internal integrator that converts the input voltage to current pulses that are applied directly to the input via the electrode capacitance compensation capacitor in the headstage (this is referred to as a freqtest input, since one use is to set or verify the frequency response of the system). A 10V voltage step will give a 10nA current step. The capacitative coupling means that this input is suitable for AC signals only, but the full-scale current amplitude extends down to 100Hz. This facility is very useful, but it is normally used in conjunction with the internal oscillator, and an external input will rarely if ever be needed.
FreqGen Out
The output from the internal oscillator, at 1V full scale, i.e. ten times higher than the internal signal level.
Command/100 In
Signals here are attenuated 100 fold, and are always added to the command signal (command potential in voltage clamp or command current in current clamp).
Command x 10 Out
This output is at x10 gain, and does not contain any bath and junction potential offsets.
Command/10 In
Signals here are attenuated tenfold, and are always added to the command signal.
Headstage Command x 10 Out
This output is also at x10 gain, and includes bath and junction potential offsets in voltage clamp mode, plus the effect of any precharging that may have been applied.
Hold x 10 In
An input that is attenuated tenfold, and which is added to the command signal only when the hold switch is in the ext position.
Cap Out
This output represents the setting of the cap control, at 10V full scale. When the RC enable switch is in its track position, the output from the automatic compensation circuitry is added in as well, so this signal normally represents the TOTAL membrane capacitance. Since the automatic component can also be as much as + or -10V, there is a theoretical possibility that the output voltage limit of about 14V will be reached, although the automatic compensation circuit will still work correctly under these conditions, and the automatic component can still be read from the imaginary phase output (see below). Note that correct interpretation of the output voltage requires knowledge of the settings of the capacitance range and capacitance offset switches, and also depends on whether the Optopatch is in small cell or big cell mode, but these parameters can be read and/or set via the computer connector.
Cond Out
This output represents the setting of the series cond control, at 10V full scale. The same remarks apply (where appropriate) as for the cap output.
Imaginary Phase Out
An output that is a measure of changes in membrane capacitance, provided by the lock-in amplifier. When the RC comp switch is on but not in the track position, it is affected directly by the gain setting, and at unity gain a 1V rms sine wave gives a DC output of 1V. When the RC comp switch is in the track position, an output of 1V represents a change in capacitance of 10% of the full-scale value of the cap control. The output voltage is independent of the gain setting in this mode, but the gain setting affects the response time of the feedback loop. Higher gain quickens the response, up to a limit determined by system stability considerations, but response times well below 100msec can easily be achieved under most conditions. Thus in track mode, one can either read the capacitance changes from this output (preferred for high-resolution measurements) or the total capacitance from the cap output.
Real Phase Out
An output that is a measure of access (series) resistance, provided in conductance units. The same remarks apply as for the imaginary phase output.
Cap Dither In
An input that is active when the phase switch is in the dither position. A 1V input changes the effective setting of the cap control by 1% of full scale. Note that, to simulate an INCREASE in membrane capacitance, a positive input REDUCES the control setting by that amount. Negative signals can also be applied.
Cond Dither In
This input is basically the counterpart of the cap dither input, and operates in the same way, but it is also associated with the automatic phase compensation system. When the phase switch is in the dither position AND the RC comp switch is in its track position then a low-frequency sinusoidal dither signal applied here will cause the switching phase of the lock-in amplifier to be adjusted so as to minimise the level of this signal on the cap output. This signal can be generated by an internal 70Hz oscillator, and its amplitude can be varied by the cond dither level control on the rear panel. When the oscillator is in use, the 70Hz output signal is available on this socket. To allow an external input to be used instead, the cond dither level control should be rotated fully anticlockwise to operate the built-in "off" switch.
Gain Telegraph Out
The output voltage here represents the overall gain setting. The values have been selected for compatibility with other commercial equipment. An output of 2.5V represents a sensitivity of 1mV/pA in voltage clamp mode and 1mV/mV (unity gain) in current clamp mode (although the minimum gain in current clamp mode is actually ten, since the membrane voltage is already amplified by this amount before being sent to the gain selector). Each gain increment, in a 1-2-5-10 sequence on the gain selector, increases the output by 0.5V, and the current range of the headstage is taken into account in computing the output in voltage clamp mode.
Filter Telegraph Out
This output represents the setting of the filter frequency control, at 10V full scale. It consists of one component of 0, 2, 4, 6 or 8V, corresponding to a frequency range of 1Hz, 10Hz, 100Hz, 1KHz or 10KHz, to which is added another component of between 0.2 and 2V, proportional to the frequency multiplier of x1 to x10 set by the frequency value control. For example, a frequency of 3KHz gives an output of 6.6V.
Headstage Out/Gain In
The current output (or voltage output in current clamp modes, which is actually ten times the membrane potential), directly from the headstage. If required, this socket can be switched to provide an independent external input to the gain and filter section of the Optopatch, to allow it to be used separately if required. This input should be driven from a low-impedance source.
Gain Out
The same signal as above, but after amplification according to the gain control setting.
25KHz Out
The same as the gain signal, but after three-pole Bessel filtering at a fixed frequency of 25KHz.
Filter Out
The output signal after four-pole (or eight-pole with internal accessory board) Bessel filtering at the chosen frequency.
