Getting Started with Anabit's Precision Multimeter

Precision Multimeter summary:

Anabit’s Precision Multimeter (PMM) Reference Design can deliver 6.5+ digits of measurement resolution like high cost benchtop DMMs. It has a voltage measurement range from +/- 200V with microvolt-level insight. The Precision Meter design supports a full suite of measurements, including:

• DC & AC voltage
• DC & AC current
• 2-wire and 4-wire resistance
• And your own custom measurements

The Precision Multimeter (PMM) is powered from a single input supply ranging from 3.3V to 5V and is easily interfaced with a microcontroller or microprocessor via SPI communication and simple digital logic connections.

Features and Technical Notes

• The heart of the PMM is a 32-bit Sigma-Delta ADC with > 26 bits ENOB — insane measurement resolution!
• Includes 4 built-in software controlled voltage measurement ranges (+/- 200mV, +/- 2V, +/- 20V, +/- 200V) to maximize your measurement resolution. The input has overvoltage protection up to 220V in any range.
• Built-in 10 range current source (50 μA to 3 mA) — enabling precision resistance measurements without extra hardware
• Second 24-bit Sigma-Delta ADC — perfect for 4-wire resistance or custom measurements
• High-res precision shunt — measure current from 2 amps down to milliamps with ease
• Includes a precision high stability 2.5V reference, with an optional 2.048V version for with even better accuracy and stability specifications
  

Settings and pin description

Looking at the measurement connections and current source outputs in the blue diagram below:

• The Vin- and Vin+ connectors are intended for DC and AC voltage measurements as well as resistance measurements. These inputs can be accessed from the banana connectors or the optional screw terminal connectors.
• The ISource1 and ISource2 2.54mm pin headers can be used as current sources for doing resistance measurements or for resistance based sensor measurements like RTD temperature sensors.
• The Sense- and Sense+ connectors along with the built-in second ADC can be used for 4-wire resistance measurements to compensate for cable or wiring resistance. This input can also be used for user defined custom measurements. They have a voltage input range from -2.5V to +2.5V. 

Current measurement connections and accessing the optional voltage reference shown in the yellow diagram below

• DCI2 and DCI1 are the DC and AC current measurement inputs. These inputs can be accessed from the banana connectors or the optional screw terminal connectors. They are connected across the precision current shunt (R35) via Kelvin connections. 
• The "Measurement Test Points" behind the connectors allow you to access the ADC channel used for the current measurement as well as the kelvin connections across the shunt (R35). These can be used to measure the shunts resistance value for calibration purposes or you could add your own custom current shunt.
• The 2.54mm pin headers 2.048V Ref and AGND allow you to access the option 2.048V voltage reference signal. This output could be used for resistance measurement or to measure the voltage drop of a diode.

Referring to the below diagram in pink, the Digital Power Jumper allows you to select how the digital circuits are powered and what the logic level of the digital communication is. The precision logger digital circuits can either be powered by the main input voltage (3.3V to 5V) or their is an optional second digital power input (2.7V to 5V). If you set the jumper to VIN then the main power input powers the digital circuitry as well as the analog. If you set it to VDIG a separate digital supply is used for the digital circuitry.

Let's take a look at the  power, communication, and control pins that are shown below in the pink diagram:

• the VIN: 3.3V to 5V (VIN) pin along with the ground (GND) pin serves as the main power input connection.
• The VDIG: 2.7V to 5V (VDIG) pin along with the second ground (GND) pin serves as the optional digital power input connection.
• The Ready (RDY) pin is an optional output pin from the ADC that signals when the conversion process for a measurement is complete (active low). This can be done through software via SPI communication, but this pin offers tighter timing control and can be used for interrupts.
• The SPI communication is handled by the pins: CIPO, COPI, SCK, and CS. 
• The Start (START) pin is optional and offers hardware control of starting an ADC conversion. This can also be done via SPI communication. 
• The Reset (RST) pin is optional and can be pulled low to put the ADC in a power down state. Pulling it low and then releasing puts the ADC in its default power on state.
• The CTRL pins control the reed relays on the design which sets the input voltage range. One of these pins needs to be high to create a path to the ADC, otherwise the input is open.
  • CTRL1 selects the 2V range. It is also used for the 200mV along with the programmable gain amplifier.
  • CTRL2 selects the 20V range.
  • CTRL3 selects the 200V range.

The diagram below in blue shows the switching and precision resistors used to set the input voltage ranges:

• The relays and precision resistors are used to create high accuracy voltage divider networks that set up the +/- 20V and +/- 200V measurement ranges.
• There are test points by the precision resistors that could be used to measure the actual resistor values for calibration purposes.

Precision Multimeter Example Arduino Code and Where to get Help:

There is a comprehensive example Arduino sketch available for the PMM to help you get started. The sketch uses an Arduino library made for the ADS1263 ADC IC that the PMM design uses. The open source library is available on Github (see link below) and thanks to the great and powerful Molorius for creating it. Please install the library in your Arduino environment before using the example sketch. 

Click here to access the to ADS126X.h library on Github 

Click here to access the Precision Multimeter example sketch on Github

Have a question about the PMM or run into any issues getting started with it? Please use the Anabit ADC Forum

Precision Multimeter Specifications and link to ADS1263 datasheet

• 32 bit ΔΣ ADC with up to 26.9 ENOB with built-in programmable gain amp, with second separate 24 bit ADC
• Sample rates range from 2.5 SPS to 38.4 KSPS with a maximum bandwidth of 7.74 kHz
• Built-in hardware and software calibration features to improve accuracy
• Runs off of 5V or 3.3V power source. Depending on configuration can consume up to 100mA current
• Three digital logic inputs to control precision relays to control voltage measurement readings

Link to Texas Instrument's ADS126X ADC datasheet