Boosting vertical sensitivity

The DSO Quad’s 20mV/div sensitivity is too low for me: I thought that meant the lowest sensitivity setting was 20mV/Div, not that it couldn’t see signals under 20mV.

Has anyone had any success building a high-impedance amplifier for the probes?

Update: It’s still a work in progress, but I’m having some success with an AD8055 opamp, using the “LOW NOISE, LOW POWER PREAMP” schematic from the AD8055 datasheet.

Note that to kill oscillation, I’ve found it necessary to add a 100nF ceramic capacitor DIRECTLY across pin 4 and 7 (capacitor is lying on top of the SO-8 chip, leads soldered directly to pins). That’s presumably because of the very high 300MHz unity-gain bandwidth of this chip.

As built it has a 30Mhz bandwidth with a gain of 10, which gets you down to 5mV/div sensitivity. A gain of 50 would get you 1mV/div with 6MHz bandwidth. (That’s still pretty good seeing as the DSO Quad itself has a bandwidth of about 7MHz or so.)

Note that the AD8055 doesn’t tolerate a wide supply voltage range, so my power supply is a 1K ohm/1k ohm resistive voltage divider (to generate a virtual ground) followed by 78L05 and 79L05 voltage regulators (to make +/-5V). That means it can take anything over about 14V on the power supply input; either 12x NiMH batteries or a wall wart work. A boost power supply might be nice, but might add noise too, I’m not that far yet.

Update: The values of the feedback resistors suggested in the datasheet don’t work for our purposes. Assuming a 1MOhm source impedance, the output has a DC offset of >4 volts, which drives the amp into clipping even with no signal.

Update: I got it working, using an AD8055 configured for a gain of 50 and a buffered input (using a TL081) for a 1MOhm impedance. Both amplifiers run off a +/-5V supply created from two 9V batteries by 78L05 and 79L05 regulators preceded by a 1K/1KOhm resistor rail splitter.

Grounding is very touchy with this setup: all ground connections basically need to be wired to a single point in the center of the board. (I used standard protoboard)