Testing gain of 10,000x amplifier

Note: There is a circuit for this purpose in the shop, in a magazine bin called "Test circuits", in a bag called "Multichannel Systems MPA8I test circuit". See a photo of the assembled circuit here.

Our Multichannel Systems amplifier has 64 programmable outputs, 32 of which are for local field potentials (LFPs), while the other 32 are spikes (SPKs). The LFPs are much larger and can be generated with a signal between 0 Hz to 300 Hz, while the SPKs are receptive from 300 Hz to 5 kHz. In order to make sure that both of these types of channels are being amplified correctly, the frequency of these channels must be measured and the gain factor can be calculated. The gain for the SPKs should be around 1,000, while the gain for LFPs should be around 100.

Sometimes the amplifier "forgets" its programmed gains, and it is reassuring to check that all is correct after any long break in use.

In order to create the system to test this gain, a function generator must be obtained (we have one in our lab space). Next, the oscilloscope, the preamp, and the breakout (or interconnect) box can be taken off the rig. BNC cables, alligator clips, and a breadboard will also be needed. Two pairs of resistors that differ by a factor of 1000 can be found using the voltmeter. (For instance, the first pair of resistors measured 12.3kΩ and 150Ω, while the second pair measured 14.8kΩ and 148Ω.)

The larger resistors measured in the 20k region will be known as R1 and R3, while the smaller values in the 2000 region will be R2 and R4. R1 and R2 as well as R3 and R4are set up as voltage dividers. R1 connects the recorded signal Vin to Vout, while R2 connects this signal to the ground signal, so the Vout measurement will give a signal following the formula Vout = (R2)/(R1 + R2).This formula shows the approximate difference needed between resistors.

Plug-in both the oscilloscope and the function generator. The function generator should be set to 2kHz. Channel 1 (CH1) of the oscilloscope should be connected to the function generator using a BNC cable that feeds into a T-adapter. Next, another BNC cable should be attached to the other side of the T-adapter, and a banana adapter should be placed on the free end of this BNC cable.

Place the first pair of resistors (R1 and R2) on the breadboard so that R1 is leading; make sure that R2 begins on the same row where R1 ends (but in a different column). Two alligator clips should be placed on the banana adapter of the free BNC cable.The side of the adapter that does not have a little rectangle on it is the ground plug, so that side should clip to the right side of the R2 resistor, while the other clip should attach to the left side of the R1 resistor. Make sure that these wires never touch each other.

Next, a probe should be attached to Channel 2 (CH2) to measure the flow of current. The frequencies of CH1 and CH2 can be determined on the oscilloscope. In order to proceed the ratios between the resistors and the channels must match. Gain of the channels will be calculated by output/input. (For example, the ratio of R2/R1 was 0.012, so the channel ratio must also be 0.012. In this example, CH1 measured 20V on the oscilloscope, while CH2 measured 240mV. Therefore, the gain between these two channels is the output from the probe, 240mV, divided by the input, 20V, in CH1. Gain = 240mv/20V=0.012.)

When these ratios are equal, the probe can be removed and a BNC cable can be placed in CH2. This cable will be used to connect with the breakout box channels. Additionally, the second pair of resistors (R3 and R4) can be added to the breadboard; R3 will begin in the same row that R2 ends (in a different column), while R4 will begin in the same row as the end of R3 and then go back so that its end aligns with the row that holds the end of R2 and the beginning of R3. Again, make sure that none of these wires are touching. Using another BNC cable, add a banana adapter and put an alligator clip on the non-grounded side (the side with no rectangle). Clip this cable to the R3 and add another banana adapter to the other side of the cable. Again, place an alligator clip on the non-grounded side (the side with the little rectangle). Find a wire that will fit into the connectors of the connector box and attach it to this clip. (There should be wires that will work in the coffee jar).

Next, plug in the preamp. Make sure there is an input plug connected from the breakout box to the front of the preamp and that there is another plug that connects the connector box to the back of the preamp. The connector box should have a small black wire attached to the end, which will ground it. It can be placed in the Ext Trig plug on the oscilloscope in order to ground the box.

When the breakout box is set-up upside-down, the SPK channels will be on the left and the LFP channels will be on the right. There will be 16 channels on each side to test, and they will start from the right side of the row closest to you. These cable numbers will be recorded by hooking up the BNC cable in CH2 of the oscilloscope to each channel in the breakout box.

Each cable number in the breakout box corresponds to a particular connector number within the connector box. (This can be further illustrated in the attachment below.) The free end of the wire attached to the resistors that feeds back to CH1 will be placed inside of the connectors in order to generate the appropriate signal. Each connector outlet will have two rows and a total of 15 connections, where 1 will be on the right side of the top row and 9 will be on the right side of the bottom row. The first connector outlet in the box will correspond to channels 1-8, while the second connector outlet in the box will be for channels 9-16.

As shown in the attachment, cable channel 1 will couple with connector number 2 in the first connector outlet, cable 2 with connector 10, cable 3 with connector 3, cable 4 with connector 11, and so on. Channels 9-16 can be coupled in a similar manner by using the second connector outlet.

With the wire from the apparatus made with CH1 in a connector and the cable in the breakout box directly linking to CH2, both of these frequencies can be determined on the oscilloscope. The frequency for CH1 must be multiplied by the ratio of the channels and the ratio of the resistors in order to calculate the input value. (For example, if 8V were seen on the oscilloscope as CH1 frequency, the input would be 8V x 0.012 x 0.012, which equals 0.001152V.)

The CH2 frequency is the output itself, but it is important to note that it is in millivolts (mV), so it will have to be converted into volts (V) to calculate the gain.

Gain = output/input (Example: CH1 Frequency = 8V. CH2 = 1400mV. We know from above that the input is 0.001152V, so (1.400V / 0.001152V) = 1,215.3 = Gain.)

Test each connector-cable pair and make sure that the gain factor is approximately the same. The table below shows findings from June 2010 in which the SPK values ranged from a gain of 1,128.5 to 1,302.1 and the LFP values ranged from a gain of 130.2 to 138.9.