Analog front-end of the voltmeter should provide the following functionality and properties:
This feature can be easy implemented with a comparator, voltage divider and several relays after the input op amp:
In the shown above LTSpice simulation, the comparator LT1011 have Vref = 1.25V set to the positive input. When the input voltage (the green line in the simulation) reach the Vref, the output of the comparator (the blue line in the simulation) becomes in logical low level and the switches SW1 and SW3 are closing, SW2 is opening. The output voltage (the red line in the simulation) will be connected in the middle point of the voltage divider. In result the output voltage never goes above the Vref limit. The output from the comparator must be read out from the micro controler in order to multiply the measured voltage from the ADC with the voltage divider ratio.
When input signal is bipolar, window comparator should be used as this is shown in the following simulation:
- Over-voltage protection.
- Voltage range selection (manual or automatic).
- Handling the negative voltage if the ADC analog inputs accept only positive voltage.
- High input resistance.
- Low thermal EMF connection to the test leads when low voltage levels are measured.
All electronic components for the analog front end should be carefully selected to avoid performance degradation of the ADC parameters like the noise and non-linearity.
Over-voltage protection
I was able to find two ways for over-voltage protection design:
- Using Schottky diodes: When the input voltage (the green line in the simulation) is above the positive power supply voltage (V+), the Schottky diode connected to the V+ is opening and it's forward current (the red line in the simulation) is limited from the protection resistor. In result the output voltage (the blue line in the simulation) will be clamped to value equal to the power supply voltage minus the forward voltage of the Schottky diode. When the input voltage becomes negative, the Schottky diode connected to the negative power supply voltage (V-) will be opening and will limit the negative overvoltage. LTSpice simulation can be found here.
- Using PTC thermistor in series to input and Metal-Oxide-Varistor (MOV) in parallel to the input. The MOV resistance depends on input voltage: when the voltage is above the clamping value, the resistance going low which form a short circuit. The short produce heat in the PTC which increase it's resistance and this opens the circuit and protect the ADC input.
Unfortunately both methods have the following drawbacks when high accuracy voltmeter has to be built:
- Standard Schottky diodes have relative big reverse leakage current, which influence on the op amp voltage offset. The worst thing is that the leakage increases with temperature very quickly, thus output voltage will depends on temperature controlled voltage offset. There are silicon carbide Shottky diodes which resolve this kind of problem, but they are difficult to buy and the forward voltage is greater then the internal ESD diodes. The lowest leakage schottky diode which I found was PADx series from Vishay/Siliconix which are not produced anymore. They have between 1 and 100 pA maximum reverse leakage current. Few replacements exists from the following companies: InterFET (DPADx), Firechild Semiconductor (FJH1101), Central Semiconductor Corp. (BAV45).
- Using PTC and MOV decrease the overall input resistance, because the MOV is connected in parallel to the op amp input and it's resistance is a few MOhms.
- Using PTC and MOV decrease the overall input resistance, because the MOV is connected in parallel to the op amp input and it's resistance is a few MOhms.
Nice video for general multi-meter protection made David Jones from EEVBlog. It include also current range protection as well.
Fortunately during selection of input op amp, I found LT1167 op amp which can handle up to 100V only with current limit resistor 5K in series to the input. This op amp has nice features: 200/1000 GOhms min/typ input resistance, sub-nA current bias, 1/6 ppm typ/max gain nonlinearity and output offset trimming possibility for calibration.
Automatic Voltage range selection
Most of the ADC have reference voltage below 5 V and a special circuit should be implemented to put the input voltage within the ADC range.This feature can be easy implemented with a comparator, voltage divider and several relays after the input op amp:
In the shown above LTSpice simulation, the comparator LT1011 have Vref = 1.25V set to the positive input. When the input voltage (the green line in the simulation) reach the Vref, the output of the comparator (the blue line in the simulation) becomes in logical low level and the switches SW1 and SW3 are closing, SW2 is opening. The output voltage (the red line in the simulation) will be connected in the middle point of the voltage divider. In result the output voltage never goes above the Vref limit. The output from the comparator must be read out from the micro controler in order to multiply the measured voltage from the ADC with the voltage divider ratio.
When input signal is bipolar, window comparator should be used as this is shown in the following simulation:
When more than 12 V input is applied to the voltage range selector, circuit won't work. How did you solve this?
ReplyDeleteHello,
ReplyDeleteThis simulation was for the case when you have 1V and 10V range DMM. The maximum range of the input signal depends on the voltage supply and the input voltage range of the input op-amp. Over-voltage protection must be used for input signals above the op-amp voltage supply.
In the case of the LTC1167, the absolute maximum voltage supply is -/+ 20V, but the datasheet on page 3 specified only input voltage range for +/-18V supply to be +1.9V for negative rail (max input signal -16.1V) and -1.4V (max input signal +16.6V). If -/+ 15V voltage supply is used, the input signal limitation will be -13.1V and 13.6. So in theory, the input signal can be up to these values. Can you tell me if you tried to make this circuit in reality and it does not worked as expected?
Please, check also the post named "Breadboarding automatic voltage range". It gives schematics for 2/20V range. At the time of testing, I do not had high voltage version LTC2057HV for the first U1 op-amp. Because of this, the voltage supply in schematics is set to -/+ 15V. If you put LTC2057HV and power supply above -/+24V you will have 2/20V range capability with some over range. Again, input signals above the voltage supply of the input op-amp must be handled by the over-voltage protection circuit.
If explanations are not clear, please contact me to the e-mail address given in the blog profile.
Kind Regards,
Krasi