VM110N Analogue Inputs

Modules Microcontroller Programmer - Experimenting Project
1

Just a quick question.

I can change R8/R9 on the K8055/VM110 board to give me x4 or x15 gain or by selecting the correct resistor value I can choose a different multiplier?

I assume I can also use a gain of less than 1 (attenuation) in order to scale inputs that are too high?

I have a system based around 24v lead-acid batteries that has a digital voltmeter across the batteries and a digital ammeter fed from a shunt in series with the -ve battery terminal. Currents from 0 to 20A are represented by a 0 to 0.2v voltage across the shunt. With a gain of x20, the analogue input can happily resolve this current reading within the 0-5v input range of the analogue input.

If I attenuate the second input, my battery voltage can be happily measured using a gain of 0.1 (/10).

This is fine for logging instantaneous current and voltage readings during discharge, but if the batteries are under charge conditions (output circuits turned off and batteries charging from solar panels), the current is flowing INTO the batteries and the shunt will therefore show a negative voltage of between 0 and -0.1v maximum.

Does the VM110 support -ve analogue voltages, or will I need to use an op-amp configured as a voltage adder in order to apply a shift or offset?

2

[quote]I can change R8/R9 on the K8055/VM110 board to give me x4 or x15 gain or by selecting the correct resistor value I can choose a different multiplier?[/quote]Yes, this is what you can do.

[quote]I assume I can also use a gain of less than 1 (attenuation) in order to scale inputs that are too high?[/quote]You can use the trimmer potentiometers (ATT1 and ATT2) to attenuate the input voltage to proper level.

[quote]Does the VM110 support -ve analogue voltages, or will I need to use an op-amp configured as a voltage adder in order to apply a shift or offset?[/quote]The board supports only positive input voltages. Indeed, you can do some arrangement to shift the voltage on positive side.

3

Without R8 installed, OP-AMP IC1A acts as a voltage follower.

Installing R8 completes the voltage divider with R10, that drives the negative feedback to the OP-AMP. That changes the OP-AMP configuration to an actual amplifier. With the correct resistor you can set it to pretty much any gain factor greater than one.

The exact same is repeated with R9+R11 and IC1B.

4

Aha…and then subtract the offset via my software to recover the ‘sign’ of the signal. I’ll give it a try :bulb:

5

[quote=“retrotecchie”][u][color=#000000]Just a quick question.

I can change R8/R9 on the K8055/VM110 board to give me x4 or x15 gain or by selecting the correct resistor value I can choose a different multiplier?

I assume I can also use a gain of less than 1 (attenuation) in order to scale inputs that are too high?

I have a system based around 24v lead-acid batteries that has a digital voltmeter across the batteries and a digital ammeter fed from a shunt in series with the -ve battery terminal. Currents from 0 to 20A are represented by a 0 to 0.2v voltage across the shunt. With a gain of x20, the analogue input can happily resolve this current reading within the 0-5v input range of the analogue input.

If I attenuate the second input, my battery voltage can be happily measured using a gain of 0.1 (/10).

This is fine for logging instantaneous current and voltage readings during discharge, but if the batteries are under charge conditions (output circuits turned off and batteries charging from [/color][color=#000000]solar panel[/color][color=#000000]), the current is flowing INTO the batteries and the shunt will therefore show a negative voltage of between 0 and -0.1v maximum.

Does the VM110 support -ve analogue voltages, or will I need to use an op-amp configured as a voltage adder in order to apply a shift or offset?[/color][/u][/quote]
Hello friend have you use the op-amp configuration? I do have similar plans but I need some serious information about this… Hope you will help me out…

6

Sorry Shawn, it’s been a while since I visited this forum, so I missed your post.

Here’s the deal. I have a pair of LM335AZ temperature sensors connected to a differential thermostat to control the circulator pump on a solar thermal system. It’s a standalone stat and works very well, but I have no actual readouts of temperature of the solar panel or the water tank.

To this end, I decided to use the K8055/VM110 to measure the temperatures of the two sensors at periodic intervals, display actual temperatures in Celcius and to log those readings to a text file that I can later import into a spreadsheet to play with the data.

The LM335AZ outputs 10mV/K and gives 0v at absolute zero, 2.73v at 0 Celcius and 3.73v at 100 Celcius. While the sensor output is well within the range of the analogue inputs, it means that my resolution is pretty poor. It works out at something like 1.9 Celcius per step with an 8 bit ADC. In the real world, I’m not likely to have a range below perhaps 0 Celcius at minimum (except in a very harsh winter) and 80 Celcius at best, on a nice hot summer’s day.

So, instead of reading a narrow band between 2.73v and perhaps 3.5v and limiting myself to a fairly poor resolution (not just the resolution…the accuracy of +/-1 i bit means a potential swing of almost 3.8 Celcius for any given reading) I want to get the whole of my temperature range as close as I can to the full range (0-5v) of the ADC.

A unity gain differential op-amp circuit with a ‘reference voltage’ of 2.6v or so on the inverting input and my sensor voltage on the non-inverting input will give me an input into the ADC of about 0.13v at 0 Celcius (giving me a little leeway for the occasional below freezing temperatures in winter) and just over 1v at maximum temperature of around 80 Celcius. I can then go for the gain of X4 on both channels to give me a much better swing of around 4v for my temperature range, reducing the +/-1 bit error and increasing the resolution.

My question now is, while I can happily add R8 and R9 to the ‘full size’ K8055, can this be done with the VM110N? I can’t seem to see any through-hole pads on the VM110N to incorporate the gain resistors. Dabbing a ‘flying’ resistor between the appropriate op-amp pins and ground isn’t impossible, but it’s not the most elegant solution.

Alternatively, I may just have to resort to the larger K8055 as these resistors can be added neatly and retrospectively. My ‘sampling time’ is ten minutes…solar thermal doesn’t have sudden changes…so the slower response of the original K8055 isn’t a problem, apart from the DirectInput issues.

7

Actually, I’ve just answered my own question. There are pads (R7 and R13) to add gain resistors, but these are fiddly little SMT components that a) I can’t buy just a few at a time and b) at my age and with my eyesight, it’s a no-no!

Plan B…add the gain I need onto my differential amplifier input stage!