function current = vstim (voltage,scaling,gain,frequency,varargin);
%
% VSTIM
%
%   CURRENT = VSTIM(VOLTAGE, SCALING, GAIN, FREQUENCY)
%                     uses the columns of the matrix VOLTAGE
%                     as analog output channels; the input/output
%                     is done at FREQUENCY.  SCALING is the command
%                     voltage sensitivity set on the amplifier (mV/V),
%                     and GAIN is the output scaling (V/nA).
%
%   CURRENT = VSTIM(VOLTAGE, SCALING, GAIN, FREQUENCY, NREPEAT) 
%                     repeats NREPEAT times
%
% input VOLTAGE is in mV
% output CURRENT is in pA
%


% The A/D output card we use only allows two output channels,
% So we will throw up all over the user if they're giving us
% a VOLTAGE array with more than two rows.

if (size (voltage, 2)) > 2
	error('Too many output channels specified!  I''m not superhuman!');
end


%
% If an NREPEAT input came, then make VOLTAGE into an
% m by NREPEAT matrix (where m was the original size
% of VOLTAGE)
%

if nargin>4
	nrepeat=varargin{1};
	voltage=repmat(voltage,nrepeat,1);
end


% Adjust for SCALING.  SCALING is specified in mV/V.  So in order
% to get p mV output to the cell, we have to output p / SCALING
% volts.

command = voltage / scaling;

% Finally, send to aoi;

aoi_output = aoi(command,[0 1],frequency);


% The output of aoi is volts.  To convert to current in pA,
% divide by the gain (V/nA), and multiply by 1000.

current = (aoi_output / gain) * 1000;

% ---------------