Handling the negative voltage
If multi-slope integrated ADC or ADC with true differential input are used in a voltmeter, there is no need for a special handling of the negative voltage.
However most of the ADC on the market supports only positive voltage input and have uni-polar power supply. In this case the analog front end should convert the negative voltage input to positive or to limit it to the minimum allowed voltage (usually limited by ESD protection diodes in the ADC to several hundred mV below the ground).
I spent a lot of research time for this part of design and found the following approaches:
However most of the ADC on the market supports only positive voltage input and have uni-polar power supply. In this case the analog front end should convert the negative voltage input to positive or to limit it to the minimum allowed voltage (usually limited by ESD protection diodes in the ADC to several hundred mV below the ground).
I spent a lot of research time for this part of design and found the following approaches:
- Full- or half-wave rectification of the input bipolar signal.
- Switching between none inverting op amp when signal is positive and inverting op amp when signal is negative, using comparator and relays.
- Setting negative voltage on the ADC's ground pin and level shifting the digital output pins. This approach can be seen in the LTC2442 datasheet, page 27/28 and can be used also for LTC 2440 according LT tech support.
- Level shifting of the input signal.
- Bipolar single ended to differential conversion.
The half-wave rectification is based on the over voltage protection schematics and it is the most cost effective variant. LTSpice simulation can be found here.
The current limiting resistor should be selected in this way that voltage across the Schottky diode should not exceed maximum allowed input voltage for the ADC. In this example this is -0.3V. The drawback of this solution is that the voltmeter will detect the negative voltage, but will not able to measure it.
An example for the full-wave rectification can be found in the LT1001 datasheet, on bottom of the page 9, where schematics of "Precision Absolute Value Circuit" is shown. LTSpice simulation can be found here.
This schematics have the following drawbacks:
- Requires 5 matched resistors. This is costly and very hard to achieved.
- Requires 2 op amp in the signal path. This will increase the noise.
- In the mV range, distortion of the signal can be seen in the simulation:
The second option relies on swapping input signal between two op amps using relays. LTSpice simulation can be found here.
This schematics have the same output like the full-wave rectification, but required only 2 matched resistors (LT5400-1) and 2 SPDT relays. When the input voltage is below the ground, the comparator will close relays for inverting op amp and will open those for none inverting op amp.
The next approach is the level shifting of the input signal. LTSpice simulation can be found here.
This method shift the input voltage using half of the reference ADC voltage.
The schematics use one op amp for the shifting, one for Vref buffering and 2 quad LT5400 matched resistors.
The last method is single to differential conversion. This can be done with specialized op amp with differential outputs or with discrete op-amps.
If the ADC have to be buffered, information can be found in the comprehensive application note named "Buffer Op Amp to ADC Circuit Collection" from the Texas Instruments company. Also good source of information can be found in the Analog Devices's "The Data Conversion Handbook", chapter 6, Interfacing to Data Converters.
The current limiting resistor should be selected in this way that voltage across the Schottky diode should not exceed maximum allowed input voltage for the ADC. In this example this is -0.3V. The drawback of this solution is that the voltmeter will detect the negative voltage, but will not able to measure it.
An example for the full-wave rectification can be found in the LT1001 datasheet, on bottom of the page 9, where schematics of "Precision Absolute Value Circuit" is shown. LTSpice simulation can be found here.
This schematics have the following drawbacks:
- Requires 5 matched resistors. This is costly and very hard to achieved.
- Requires 2 op amp in the signal path. This will increase the noise.
- In the mV range, distortion of the signal can be seen in the simulation:
The second option relies on swapping input signal between two op amps using relays. LTSpice simulation can be found here.
This schematics have the same output like the full-wave rectification, but required only 2 matched resistors (LT5400-1) and 2 SPDT relays. When the input voltage is below the ground, the comparator will close relays for inverting op amp and will open those for none inverting op amp.
The next approach is the level shifting of the input signal. LTSpice simulation can be found here.
This method shift the input voltage using half of the reference ADC voltage.
The schematics use one op amp for the shifting, one for Vref buffering and 2 quad LT5400 matched resistors.
The input signal is inverted by the first op amp and level shifted from the positive input from buffered ADC voltage divided by 2. The output voltage is additionally divided by 2: from 2.5V peak to peak to 1.25V peak to peak and the lowest voltage is 0V.
The last method is single to differential conversion. This can be done with specialized op amp with differential outputs or with discrete op-amps.
If the ADC have to be buffered, information can be found in the comprehensive application note named "Buffer Op Amp to ADC Circuit Collection" from the Texas Instruments company. Also good source of information can be found in the Analog Devices's "The Data Conversion Handbook", chapter 6, Interfacing to Data Converters.
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