Archaeology Resistivity Meter

Great, thanks.

Might be worth adding:

the ADC type - PCM4201

note "overload monitor" on the analogue signals going into the level shifters from the input and output amps (I forgot them on the second diagram)

MK

Hi Ken and Michael,

Nice diagram!

Looks good.

Do you think we need a spec next ?

Might be worth hanging on a couple of days to get comments on the block diagram and thoughts so far.

Things to think about on spec:

Operating temperature range

Endurance on one fully charged battery

Max output current and voltage (if 20mA RMS and nearly 60V pk-pk won't do it's going to get a lot more expensive )

Input common mode range (max +/- 14.5V including signal) ref instrument internal ground.

Max signal combined signal and common mode +/- 29V differential

Input impedance - could be 100M if wanted but I think this is a bad idea - should be lower , maybe 1M - problem is that you want the option

of input caps for AC coupling. If you have 100M input Z then you get -3dB at 10Hz with a 160pF input cap, but its impedance at

150Hz is 6.6M which will make things noisy. I'd like the capacitor impedance to be no more than 50k at 150Hz = 22nF, with 100M input that gives

a time constant of 2.2s so it may take ages to settle, I'm tempted to go for 10M or even lower input resistance to reduce that effect. (NB, the settling time

of the AC coupling RC is nothing to do with the normal settling time of measuring - its to do with how long it takes the system to settle after a large DC

bias is applied to the input.) Ideas / comments etc welcome. (AC coupling will be switchable.)

Input protection - static discharge (8kV ?, 16kV ? - full lightning ? (please not !) - any ideas from the field.

Operator controls - buttons - membrane key pads are expensive - nice push buttons are also expensive -  any ideas.

Would a touch screen be any good in the field ?

Well there's a good collection of random thoughts and comments.

MK

Ken,

I'm just catching up, sorry to hear of your loss.

Michael, will post some feedback on various points in a moment

Dave

Michael,

Quick observation on the block diagram, ADC monitors voltage probes, but one thing that I cannot spot is provision for measuring/monitoring the injected current? If running in mode of conventional resistivity kit (square wave with constant current) then need a way to set and then monitor; if square wave with constant voltage, or a non-square-wave other waveform, need to continuously measure current to compute res?

Dave

Well, sort of yes and no, the output amplifier is a current source, (see posts 42 - 44 in this thread), so the amplifier input is a voltage from the DAC and low pass filter

but it forces a current, proportional to the input voltage, into the load. If you try an force too much current the amplifiers might not be able to generate a high enough output

voltage, which is why there is voltage feedback to the processor analogue ports (which will use the processors rough and ready ADC to keep a check that the voltage

isn't too high).

So the output current is continuously referred to the demand voltage, by the nature of the feedback arrangement of the amplifier. (If you are interested in looking more deeply into this

try Googling "Howland Current Pump", my circuit is a differential version of that venerable design.)

The block diagram shows a digital gain control from the processor to the output amplifier, by switching the current sense resistors and  varying the codes to the DAC we will be able

to set the current anywhere between 20 mA and 50uA with reasonable accuracy and good repeatability.

MK

Hi Michael,

I had these initial thoughts, but I've not had time to think things through (I have some time late Wednesday to do that):

There was mention of more sense probes, but maybe the expansion interface is sufficient for that, since one could drive (say) a latching relay via that. Or maybe space for on-board relay?

A minor point was, for the USB UART, for MCP2221 I already have Android code. But FTDI is totally fine too, i.e. if you've already got the schematic entered for that portion then it's quicker for me to just hunt for demo code for FTDI, if you let me know the chip number. Below is the the schematic I've used previously for the MCP2221 (it needs an external 3.3V regulator which probably the FTDI device doesn't, and has 4kV protection which likely isn't enough, I didn't place any additional ESD protection on this schematic).

One other thought was for a test facility, if there was were four jumpers or four DIP switches on the board which would connect a potential divider across the source (two switches for that) and two switches to connect the sense wires to the divider. Maybe jumpers are easier. But probably there will be test points on the board for easy soldering of a mini test harness during software development anyway.

Hello Sabaz,

I'd prefer the FTDI chip (FT230X) since I've used it before and had no problems - no regulator needed.

It doesn't have good ESD protection either.

I'll look at the Microchip part.

I usually design boards with lots of test points , compatble with those little wire loop 'pins', or wires, since it's a pad with a 1mm hole.

The full processor debug will be available and at least two FPGA pins.

I'm leaving mpx till a later design and additional pcb. If the expansion interface carries some signals and power it can do almost  anything.

MK

I've looked at the Microchip part, very little to choose between it and FTDI, Microchip possibly better documented.

I like both companies - so I'll go either way !

BATTERY

I have results form first battery test.

These are nominal 7.4V 8.8Ah batteries from Amazon at £40 for two including a USB powered charger.

Not an ideal way to power a dual charger for these batteries - it would be quite safe to charge them at 2A max each

taking about 5hours, charging at less than 0.5A each is going to be slow.

My electronic load can do a simple (constant current load) battery rundown test so I set it for 0.5A load and

5.9V end point. The battery voltage has bounced back a little once the load was removed, which is not unusual.

I've recorded 7.527 Ah, which isn't that bad for a dead cheap battery rated at 8.8Ah. Its fine for this application.

It would be better to test at constant power, but the Rigol can't do that.

I might have squeezed a bit more by ending at 5V (2.5V per cell) but experience tells me that there won't be much

energy left between 2.95 and 2.5 volts per cell and it's not good for long life to repeatedly discharge to that level.

I've got the battery on charge and I'll run it again, then I'll try the other one.

I estimate that I got 54.2 Wh out of it - I'm looking for  a minimum of 30Wh for this application, so unless it dies

very quickly it looks OK.

MK