7106s, PICAXEs, and op-amps

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Andrew Cowan

Senior Member
The 7106 IC (A/D convertor) is the core of most cheap digital multimeters. The two factors that make it so useful are:

1 - Very low input current needed for a A/D reading (ie high input impedance).

2 - A VERY high maximum voltage allowed on input pins. The datasheet states Input voltages may exceed the supply voltages provided the input current is limited to ±100μA, but offers no further details. This means that you can measure a 1000v DC signal with the multimeter on the 1000:1 divider, but using the 200mV setting (no divider) will not damage the multimeter due to the high voltage allowed.

Creating a PICAXEd solution for the flexibility of measuring voltages

While the PICAXE can accept high voltages while current is limited, it does not have a very low input imedance. This means some kind of external buffer is needed. An op-amp is the usual answer. However, I haven't been able to find any op-amps with a high maximum input voltage.

My answer

My thought would be to use a 1:1 op-amp to buffer the signal, into a PICAXE ADC port. Under normal conditions, the input voltage to the op-amp would not be above the op-amps maximum. However, when the voltage does go above (wrong mode selected), a MOV rated at 12v or so would switch on, keeping the voltage low. The PICAXE would detect the voltage was at the MOV's trip voltage, and indicate the wrong mode was selected. Obviously there would be a resistor to limit the current flow to the MOV.

Would this setup work or have any problems (eg voltage loss)? The lowest rated MOV I have found is 12v.

I'd be interested in any thoughts.

Andrew
 
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BeanieBots

Moderator
MOVs can often have sharper knees than zeners but it's still not a good idea to put non-linear devices in linear analogue networks and expect a linear response.
The safest 'simple' solution is to use a low leakage clamp diode.

Failing that, design your input divider such that at your maximum input voltage, no input maximums are compromised. Then design your amplification stage to cope with the lowest input range. Then everything will be happy.

Have a good read of the MOV datasheet. What is its leakage current? What is its IV curve?
How will that effect your divider accuracy?
 
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B

BeanieBots

Moderator
It's all about DESIGN and experience.
Hippy is correct that high impedances and high gain are an easy route for noise. Think about what happens if you touch the phono input on your hifi with a finger!
Just like your hifi, a high gain high impedance input to a PICAXE ADC needs to be designed and constructed in such a way that noise does not get picked up.

Equally, clamp diodes, zeners and MOVs all have leakage currents, capacitance and even generate noise themselves. Diodes are actually used as noise generators. So are carbon resistors.

This is where datasheets and calculators come into the equation.
Quality analogue work requires quality components and they cost money.
 
M

MFB

Senior Member
I would suggest using a rail-rail amp configured for non-inverting unity gain. This provides a high input impedance for the potential divider at its input. Use resistor values that are high enough to protect the opamp if/when its input is driven outside it’s supply range. Provided that this supply is the same as the PICAXE, the ADC will not be overdriven by an opamp input overload. Regarding noise, never use more bandwidth than you really need. Therefore, add a capacitor to the centre of the potential divider/opamp input to act as a low-pass filter.
 
fernando_g

fernando_g

Senior Member
Linear Technologies has a line of "over-the-top" opamps, whose common mode input range far exceeds the supply voltage.

You can measure up to 44 volts input while the opamp is fed from a 5 volt supply, which would satisfy a large variety of voltage-monitoring apps.....but of course, not all the way to 1000 volts.
 
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Andrew Cowan

Senior Member
Well, I've never needed to monitor any DC voltage close to 1000v.

Use a 10:1 divider, have the op-amp multiply by 10, and you can measure up to 400v without much loss.

Thanks for that.

A
 
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