Getting Started with Anabit's Precision Logger
Precision Logger summary:
Anabit's Precision Logger is an ultra-high resolution, open-source ADC / data logging solution that delivers microvolt-level insight with lots of analog flexibility. The Precision Logger's 32 bit ADC combined with its outstanding accuracy specifications allows you to achieve measurements of nearly 8 digits of usable precision. This makes the Precision Logger ideal for applications where small voltage changes matter. The Precision Logger delivers multiple channels that can be configured as 5 differential or 10 single-ended inputs using the integrated analog multiplexer.
Features and Technical Notes
- 11 total flexible analog channels
- Any of the 11 channels can be paired to make up to 5 differential input channels or use up to 10 inputs as single ended channels with the 11th serving as the analog ground reference or mix and match differential and single ended
- Any channel can be routed to the main 32 bit ADC or the optional 24 bit ADC
- Input analog pins can be configured as 1 to 8 general-purpose digital inputs/outputs
- Built-in low-drift 2.5V reference, with the option to add a 2.048V reference with increased accuracy and ultra-low drift.
- Programmable Gain Amplifier (PGA): Gain settings of 1, 2, 4, 8, 16, 32. PGA can also be bypassed.
- Two precision current sources (IDAC1 and IDAC2) with 10 selectable output ranges: 50 μA to 3 mA. These are great for performing accurate resistance measurements.
- Optional secondary 24-bit ADC — ideal for cable resistance compensation for sensors such as RTD
- Combine second ADC with the integrated temperature sensor for cold junction compensation in thermocouple measurements
- Input hardware filters and integrated Digital filters to attenuate input noise and 50Hz/60Hz power line ripple
- Digital Filter settings: Sinc1, Sinc2, Sinc3, Sinc4, Sinc5, and a finite impulse response (FIR) filter
Settings and pin description
The precision logger has some on board power settings that the user needs to be aware of. It consists of three different jumpers that configure its power input and input voltage range (see figure in blue):
- 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) to power the digital circuits and set the digital logic level voltage. If you set the "Digital Pwr Source" jumper to VIN then the main power input is used to power the digital circuitry as well as the analog. If you set it to VDIG a separate digital supply input is used to power digital circuitry.
- The other two jumpers determine if the analog supplies to the ADC and PGA are configured -2.5V and 2.5V or 0V (GND) and VIN (3.3V to 5V).
- Setting ADC Neg Supply to VSS = -2.5V and ADC Pos Supply to VDD = +2.5V enables bipolar inputs for all measurement channels with an input range of -VREF to +VREF
- Setting ADC Neg Supply to VSS = AGND and ADC Pos Supply to VDD = VIN enables unipolar inputs for all measurement channels with an input range of AGND to +VREF. The main benefit of this setting is you have a reduced voltage range so you get even more resolution but of course you cannot measure any negative voltage signals
The DIP switch, labeled as Ext Volt Ref Routing, is only included with optional 2.048V ultra-low drift reference IC and is used to route it to the ADC. External references are routed to the ADC through some of its analog input channels. The diagram below in blue shows the pins (A0 thru A3) and reference signals that the DIP switches control (this is also printed on the Precision Logger's PCB). When the DIP switches are left off or open, then those pins are available as measurement channels.
Let's take a look at the analog input pins and the voltage reference pins that are labeled in the yellow diagram below:
- Pins labeled ChanA0 (A0) to ChanA9 (A9) and ChanACOM (ACOM) are the 11 analog measurement inputs. The intended purpose of ACOM is to serve as a ground connection for single ended channels, but ACOM can be used just like the other 10 input measurement channels.
- Each of these channels connect to the MUX that is used to route them to the ADC via software control
- Each of these channel paths includes filtering to attenuate any high frequency noise on the input signal
- Pins labeled as DirectA4 (A4-D), DirectA5 (A5-D), DirectA9 (A9-D), and DirectACOM (ACOM-D) are the same MUX pin connections as A4, A5, A9, and ACOM. The difference is there is no filtering or resistors in the signal path. These direct connection points were put in to be used as current source channels.
- Voltage reference outputs and analog ground (AGND) connections
- If you have the optional 2.048V precision voltage reference it can be accessed from 2.048V Ref (2.048V) pin. It has an output drive capability of up to 10mA
- The ADCs internal 2.5V reference can be accessed at 2.5V Ref (2.5V) pin. Be aware that the 2.5V reference is referenced to VSS (not GND). That means if VSS is connected to the -2.5V power supply, the 2.5V reference will measure at or close to 0V from ground. When VSS is connected to ground then the 2.5V reference will measure 2.5V to ground.
- The AGND or analog ground pins are connected to the Precision Logger's continuous ground plane. But they are upstream of the noisy digital and power signals on the board so they are meant to be used for analog measurement purposes.
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 (VINPWR) pin along with the ground (GND) pin serves as the main power input connection.
- The VDIG: 2.7V to 5V (VINDIG) 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.
Precision Logger example Arduino code and where to get help:
There are two example Arduino sketches available for the Precision Logger to help you get started. Both example sketches make use of an Arduino library made for the ADS126X ADC IC that the Precision Logger 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 sketches.
Click here to access the to ADS126X.h library on Github
The first Precision Logger example sketch enables you to make a voltage reading (single ended or differential) from any of the input pins. It also allows you control various settings such as sample rate, digital filter, and more. Click here to access the sketch on github
The second Precision Logger example sketch enables you to make a K-Type Thermocouple measurement. It makes use of the ADC's built-in high accuracy temperature sensor to use as a cold junction compensation for the thermocouple measurement. Click here to access the sketch on github
Have a question about the Precision Logger or run into any issues getting started with it? Please use the Anabit ADC Forum
Precision Logger Specifications and link to ADS126X datasheet
- Resolution: 32-bit ΔΣ ADC with up to 26.9 ENOB
- Maximum Sample Rate: 38.4 kSPS with a maximum bandwidth of 7.74 kHz
- Main ADC high-accuracy architecture:
- Offset drift: 1 nV/°C
- Gain drift: 0.5 ppm/°C
- Noise: 7 nVRMS (2.5 SPS, gain = 32)
- Linearity: 3 ppm
- Over voltage protection on input pins up to +/- 10V
- SPI Interface: Supports clock speeds up to 10 MHz
- Optional Second 24-bit ΔΣ ADC: Max sample rate 800 SPS and built-in PGA with gains settings: 1, 2, 4 ,8, 16, 32, 64, and 128
- Optional Voltage Reference: using ADR4520ARZ
- Output Voltage: 2.048 V
- Initial Accuracy: ±0.04%
- Temperature Coefficient: 2 ppm/°C (typical)
- Output Drive Capability: Up to 10 mA