Multichannel Systems MEA1060-Up-BC Manual - Manual (Page 36)

MEA Amplifier with Blanking Circuit for Upright Microscopes
36
5.6 Stimulus Artifact Suppression (Blanking)
A TTL pulse (blanking signal) that has to be timed exactly with the stimulus pulse triggers
both the stimulation and the blanking. During the blanking signal, both stimulus inputs are
connected to the software-selected stimulating electrodes (A and B), and all MEA electrodes
are disconnected from the amplifier. The last signal value on all electrode channels before the
blanking pulse will be saved and then kept until the channels are reconnected to the amplifier.
This avoids that the amplifier gets saturated on all channels during stimulation (which is the case
with standard amplifiers). You can disable the blanking for test purposes.
The stimulus artifact suppression performance depends mainly on the electrical state of the
electrode (that is the residual net charge of the electrode). This depends on the stimulus type
(current or voltage driven, biphasic or monophasic), strength of the stimulus (amplitude,
duration, and frequency), on the electrode type and size, and other parameters and has to be
determined empirically. Complete artifact suppression may not always be possible, depending
on those parameters. Generally, the stimulus artifact suppression performance is much better
when using voltage driven stimulation than current driven stimulation. In the following, you will
find some general recommendations for an improved stimulus artifact performance, but please
understand that it is not possible for us to discuss all possible variations in this document.
Timing and duration of blanking signal
For most applications, MCS considers a delay in the range of 100 μs suitable for discharging the
electrode and recommends a minimum blanking pulse that starts with the stimulus and stops
100 μs after the stimulus. The fact that the digital output of the stimulus generator from MCS
is about 20 μs faster than the analog output ensures that the blanking signal is delivered before
the stimulus. If you use a pulse generator from a supplier other than MCS, you may have to use
other trigger settings. It is very important that the blanking pulse is delivered slightly before the
stimulus.
For improving the blanking performance, you can try to extend the blanking pulse after the
end of the stimulus pulse. This gives the electrodes a longer time to discharge before they are
disconnected from the stimulus inputs. The charge carrier cannot flow back to the STG after the
end of the TTL pulse because the connection is interrupted. MCS recommends to try out 500 μs
and then decrease the time length stepwise for optimization if you observe problems with the
artifact suppression performance. The AutoSync feature of the STG 2000 series makes it easy
to adapt the Sync Out output of your stimulus protocol.
The following example shows a blanking signal (programmed with MC_Stimulus) of 200 μs length
for a monophasic voltage pulse of 100 μs length.