“This PVIM Board is a Precision, Eight Channel, Synchronous Data Acquisition board (DVM), with Simultaneously Sampled Isolated Inputs.
Project Preamble
During various Solar energy and Battery Storage System involvements over the years, together with integration of Home Automation for various monitoring and controlling of ‘IoT stuff’, a new dedicated single board was required in order to more accurately measure multiple DC voltages.
Separate to this, and more importantly, is the ability to safely have full isolation between channels, so multiple battery banks, and strings, for example could be safely monitored using a single board.
Thus, my shopping list of features evolved for this next project.
- Accurately Measure Voltages, from a Positive and Negative Range
- Low Noise
- Good Dynamic Voltage Range
- Self-Contained Board for IoT Projects
- Minimum of Eight Totally Separate Inputs – Each Configurable
- Wide DC Input Power Voltages. Board must be able to be Powered from around 12V to 48V Directly
- Ability to Monitor Lead-Acid, Lithium Batteries. 48V Strings, or Individuals
- Ability to Facilitate Current Monitoring via CT Clamps or DC Shunts
- High Impedance Inputs
- Ability to Measure Sensitive Voltages
- Ability to Measure Temperatures (via Thermocouples)
- Ability to Store Parameters and Log Measurements to EEPROM
- Minimal Calibration Requirements
- Compact but Flexible
- Lower Current Usage
- Controlled using an ESP32 MCU
- Easy USB Connectivity Flashing, Logging etc
Key Factors
Being able to power the board from such as wide voltage range, without the issues of generating heat (so LDO’s were not an option), low noise, and availability, a LM25xx was selected. The range of devices available offer a wide input voltage up to typically 60V and around 52kHz or 150 kHz switching frequencies. The eventual device, cap and inductor values will be evaluated for low noise etc. during development.
In order to facilitate the wide measurement options, and maintain a single compact board, it was decided to dual foot print the differential inputs using a standard 5mm Terminal Block and a 3.5mm Jack Socket. Additionally, for low level measurements, where sensitivity to noise is possible, provision of coaxial unbalanced inputs was made.
With such as range of input voltage, or current, ranges, an on-board RC filter network to be included, with ability to resistive bias to 5V rail, integrate a potential divider, integrate a Burden resistor and change from balanced inputs to unbalanced inputs.
The ADC DSP Device
In researching the vast array of available ADC and DSPs which ticked my wide range of requirement boxes, together with 16bit high resolution sampling of 200 kHz (AD7606BSTZ), importantly being available in stock for SMT, the AD7606 jumped out and this now completed my Swiss-Army-Knife, Precision Voltage Monitor Board.
The AD7606 series offers a wide range of high precision analogue sampling, which could be used for so many electronic and energy monitoring projects. This board is effectively an IoT Digital Volt Meter.
The PVIM project takes this a stage further in pairing the AD7606 (AD7606BSTZ) with an ESP32 (ESP32-WROOM-32E or ESP32-WROOM-32UE), in order to provide a very useful self-contained and compact SDK board.
Data acquisition. Have designed the SDK board with potential future options. This allows for an even higher resolution version for sensitive digital voltmeter, lab measurements using the AD7606BBSTZ (16-bit ADC with 800 kSPS), or even an AD7606C-18BSTZ (18-bit ADC with 1 MSPS on all channels). However, these devices are much more expensive, specialist needs, and not required for mainstream. If you are interested though, please let me know.
Wide Range DC Power Input
In order to facilitate a wide power supply voltage range, PVIM also includes a DC to DC voltage regulator capable of operating from 8 through to 60 Volts. This makes it ideal for monitoring Solar Batteries and Home Energy Systems.
EEPROM
As with all my boards, it must have an EEPROM for east storage of parameters and logging, if required. An AT24C64 is thus included.
The EEPROM sits on I2C, so is easily accessed from the ESP32.
Flashing and Logging
The PVIM board comes complete with a ‘Wemos D1 Mini’ compatible USB interface. This enabling easier flashing and no need to have to press a button the flash.
PCB PCA
The PCB is based on 0603 components, where user configurable Resistor Capacitor filter components are 0805 size to make it easier to configure and update values for the desired users project.
The board measures only 100 x 90mm, so is quite compact and although this is a doubled sided (2 layer), board, all components are on the top side.
The normal input options are 3.5mm socket (CT clamps) etc, or 5mm screw terminal header. Each input uses a custom dual footprint which combines the two sockets and reduces the PCB size.
An IPEX U.FL RF connector is fitted. This enables readily available pig-tails to be connected (i.e. IPEX to SMA), so you have screened connections for lower level input monitoring.”