When debugging circuits, I've often found it would be useful to have an active differential probe. The vast majority of the circuits I work on are low voltage (<15v), which means the high division ratios provided by commercial probes (eg x50 and x500 for the low-cost Micsig DP10013) are not required, and attenuate the signal much more than is ideal.
As such, I've decided to design my own probe. I am aiming for the following specifications:
- 10x attenuation ratio
- Around ±30v input limits
- >100MHz bandwidth (for use primarily with a DS1054Z)
- Low cost (which means a 2-layer PCB)
- Low circuit loading (around 5Meg input impedance)
I have decided to use a standard instrumentation amplifier design, consisting of three discrete opamps (two buffers, one summing). For the opamps, the ADA4857 seems to be a good fit for my requirements, with 850MHz -3dB unitity gain bandwidth, a 2.8kV/us slew rate, and fairly low input bias current.
http://www.analog.com/media/en/technical-documentation/data-sheets/ADA4857-1_4857-2.pdfBasic instrumentation amplifier arrangementI have put together a simulation (see attached), which seems to indicates the performance should be pretty good, with a very flat frequency response, and a -3dB bandwidth for the entire probe of around 300MHz. Before I move on from simulation to designing the final schematic, it would be great to get some feedback on my design from the forum. For the actual design, the 3.3Meg resistors will probably be slit into multiple (to allow for a higher voltage variant to be made if required), and some of the tuning capacitors replaced by variable ones.
In order to avoid reflections from unterminated coax, I have designed it to be used with a 50R feedthru terminator. Obviously this means the probe itself only needs 5x attenuation (as the 50R series resistor and 50R terminator add another 2x).
Gain lineup:
- 0.216 gain in the first divider
- 1.133 gain in the buffers (adjustable via R2)
- 0.846 gain in the summing amplifier
- 0.500 gain in the series 50R resistor/coax termination
Total: 0.104 (I will fine tune R2 to get this down to 0.100)
The final stage seems very sensitive to small amounts of capacitance (C1, C10). I decreased the size of R4,R5,R6,R7 to help with this, but I wonder if a few pF of parasitic capacitance from the PCB might cause issues here. Does the magnitude of the resistors I have selected (hundreds of ohms) seem appropriate?
The AC simulation response looks great until you get to very small input signal (eg the 10mV curve attached). I presume this is due to the probe's noise floor, as even with microvolts of input this 'hump' is much the same. If this is the case, that's fine (obviously an active probe will introduce some noise) - hopefully I haven't missed anything.
I'd be interested to hear any thoughts or suggestions on the design!