Quad-SMU
AD5522 Based 4 Channel Parameter Source Measure Unit
The Blog for this project (not yet)
The Schematics, PCB files, and Simulation models are here (not yet)
Youtube Video on this project (not yet)

Introduction

As I discuss in the DIY-SMU project, SMUs are really useful. But really expensive. Evan a low voltage SMU is expensive. How about a 4 channel, low voltage SMU for about $250 parts cost? Analog Devices AD5522 to the rescue. Analog calls it a PMU (Parametric Measurement Unit) and it is typically used for IC DC parametric testing.  Since it has bipolar current and voltage source, and current and voltage measure capability, I call it an SMU. This IC is a 4 channel, 4 quadrant SMU with the following capabilities:
What would you do with such a thing?
Here is the AD5522 Quad SMU IC block diagram from Analog Devices, showing just one channel.

ad5522-1


At Teradyne in 2007-2008 I was hardware lead on a DPS or Device Power supply product. This beast was a 256 channel(!!!), water-cooled,  power supply with independent DACs and ADCs for each channel. It used 256 of another Analog Devices IC, the AD5560, a single channel Device Power Supply. This part is similar in architecture to the AD5522. I became very familiar with the AD5560. These 2 parts share a similar architecture: several 16b DACs, current and voltage ranging, power stages and similar current ranging.

The AD5522 IC  is available with two package options: Pad-up and Pad-down. It is also available on two evaluation board, one for each package. At this time (10/2024) Mouser has 133 pad-up boards and 7pad-down in stock. Digikey has no  Eval boards in stock. Digikey has about 100 of each of the ICs. Mouser has 900 pad-up ICs and a few pad-down. 

I have one of the AD5522 pad-down eval boards. I tested it with the ADI evaluation software, and it works well. I came up with a PC board design that uses an eval board. Most of the required pins are available on various headers. A few additional wires, headers and modifications may need to be soldered to the eval board. 

The AD5522 has the following on-chip:
 The evaluation board adds the following:
 The carrier board and enclosure requires the following:

The rules about using and buying evaluation boards are more restrictive than other (SparkFun, Adafruit, Ali Express...) boards. Evaluation boards built by semiconductor companies are designed and intended for protyping and evaluation purposes only, and are not for building products for sale. These are sometimes loss-leaders, meaning that they are sold at a loss in order to help engineers design in their devices. Quantity purchases are usually limited to a few units per month. Building DIY products is somewhere in the grey zone, and unless you try to buy more than a few per month, shouldn't be a problem. Now that the ICs are readily available, and if this thing takes off, I would build a proper board around the IC and eliminate the eval board.
Here is the EVAL-AD5522EBD (D for down) Eval board.

    ad5522 eval

Design

Since the core functionality of the SMU is defined by the AD5522, the project has some hardware, but is mostly firmware development

Control

For SPI data isolation. I like the Silicon Labs Si86xx series parts. For this design the 6 channel part with 5 signals in one direction and one return signal will do: Si8661. had supply issues until late 2022, the ADUM261 is a substitute, but needs an SOIC-16W wide footprint.

The power needs of this project are minimal. A few tens of milliamps of +/- 15V, and +5V. A small 1 watt, +5V to +/-15V DC-DC provides the +/-15V as well as the required isolation. A small +5V regulator makes the +5V. I have been bitten by the common-mode switching noise of DC-DC converters so to keep noise low, a common mode filter and a .01uF safety cap should help.

Here is a proposed layout in a Hammond Clamshell box, 1598D, about 8"x7"x2.4". The AD5522 eval board is below the main board and so is hidden. The +/- 15V power supply goes in the empty space. The front and rear panels would be custom designed sheet metal and built by PCBWay or other PCB manufacturer. The LCD is a Nextion 320x240 touch screen. Encoder knobs and buttons would mount above the I/O connectors.

case1 

CPU

For the CPU I haven't decided yet. It needs nothing special: USB, SCPI library, Fan PWM, serial port for the Nextion display, EEPROM for Cal.  A basic Arduino could do the job,  Currently am considering one of the SAMD21's like the Adafruit ItsyBitsy M0 at $12. If Teensy is used, the 4.x is the only available one. 

Display

There is a fair amount of stuff to display, lots of parameters. So I plan to use a 480x320 TFT LCD.  Leaning towards Nextion to get easy touch.

Firmware

The ADI GUI software that comes with the eval kit works well. Unfortunately, like most eval board software, it comes with no source code, no .DLL, and no way to use it other than with the the GUI provided. Of course, there is no source code for the Cypress embedded processor either. So you are on your own to create the low level and high level code to control this very complex IC. Oh well. At least I can hook up a logic analyzer to the SPI bus to see what the register bangs are, in case I get stuck. Better than nothing. I'll quit whining and start coding.

In addition to lots of low level and calibration code needed, here are the user controls:
 Here are the screens:

Calibration

There is a lot to calibrate: 5 current ranges and 1 voltage range on each of the 4 channels, source and measure, offset and gain, and maybe clamps. That's 4 x (5+1) x 2 x 2 = 96 cal factors, not including Clamps. I plan to use the same approach I used on DIY-SMU. An HP34401A DMM and some precision resistors, with a Python program to run it all.

The good news is that there are provisions on the AD5522 chip to output each channel to a master measurement system for calibration. That should minimize the amount of external manual or automatic switching needed. These are the SYS_FORCE and SYS_SENSE signals on the IC.

SCPI Control

For SCPI control, I plan to follow the DIY-SMU project and use the https://github.com/Vrekrer/Vrekrer_scpi_parser library. Forse and clamp settings for all channels, measure I and V for all channels. Temperature monitoring.

10/2024 Update

I got the evaluation board working again, and tested some of the features. I was able to hand-calibrate a couple of voltage outputs. Using a logic analyzer (LA1010), I could view the SPI transactions on the Eval board. I dug into the register descriptions, and was able to understand most of the bit fields. I wrote some Arduino code to drive the IC over SPI, and after a day or so of troubleshooting, was able to get reliable control. The Eval board had the LVDS termination jumpers installed. But they didn't interfere with the Evaluation board even though it wasn't using LVDS. But the Arduino didn't like them installed.  I'm currently using an Adafruit ItsyBitsy M0 module.

The OLED and barometer module are just there to see if the SPI OLED interferes with the AD5522. So far so good.
bboard



The initialization for the AD5522 only needs a few magic numbers to set up the System Control Register (SCR), the PMU registers, and Force Voltage DAC registers. Conveniently, the 4 channels can be programmed either all at once, individually, or in any combination. Without any calibration, the outputs are pretty accurate. Less than 20mV offset and +/- 0.2% gain on the +/- 10V range.

I'm working on the carrier board design and layout. It will contain the processor module, isolation, power conditioning, and the four output connectors.

Here is the list of tasks for this project.

Evaluation and Arduino Code
*    SPI Details
*    Basic Force V
    Data structures
    Force I
    Clamps
    System connects
    Guard and sense
    ADC
    Manual Calibration
    50mA range
   
Finish PCB design:
    CPU controls
    I/O Connections
    Mechanical for Eval
    Power conditioning
    Isolation
    Status LEDs
    Encode rand button connects
    Nextion serial port

Firmware:
    Controls
    Basic Nextion
    Advanced Nextion
    SCPI
    Alarms, Temperature....
   
Mechanical
    Heat Sink (isolated)
    Case base plate
    Power Supply
    Front panel PCB
    Front panel
    Rear panel

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Last Updated: 10/15/2024