Simple TIA

This circuit contains a TIA with multiple gain settings. It also contains a two channel DAC and a single ADC.

Table of Contents

• Overview
• Getting Started
  • Hardware Setup
  • Using the Shell Interface
  • Preparing for Measurements
  • Making Measurements
• Serial Commands

Features

• Singled Ended Transimpedance Amplifier (TIA) with multiple gain settings
• Configurable TIA Gain for current measurements from 1mA to 100pA
• Configurable Load Bias up to +/- 1.65V
• UART Shell Interface for configuring the DAC, Reading from the ADC and Configuring the TIA Gain.

Components

• Microcontroller: NRF52832
• DAC: MCP48FV12
• ADC: MCP3201
• Serial Port: CH340 USB to UART Bridge

Getting Started

Hardware Setup

To setup the device, plug it into your computer via a micro-usb to usb cable. And connect in the Electrode Lead Plug (DB-9) to the board.

Use a serial terminal program (like PuTTY, Tera Term, or picocom) to connect to the device. The device will enumerate as a COM port on Windows or a /dev/ttyUSB* device on Linux. The serial port operates at 115200 baud. You should be greeted with an empty terminal.

Using the Shell Interface

Once connected, you can type help to see a list of available commands.

uart:~$ help

Available commands:
  adc      : ADC commands
  amux     : Analog multiplexer commands
  app      : Application version information commands
  clear    : Clear screen.
  dac      : DAC commands
  date     : Date commands
  device   : Device commands
  devmem   : Read/write physical memory
             Usage:
             Read memory at address with optional width:
             devmem <address> [<width>]
             Write memory at address with mandatory width and value:
             devmem <address> <width> <value>
  help     : Prints the help message.
  history  : Command history.
  kernel   : Kernel commands
  rem      : Ignore lines beginning with 'rem '
  resize   : Console gets terminal screen size or assumes default in case the
             readout fails. It must be executed after each terminal width change
             to ensure correct text display.
  retval   : Print return value of most recent command
  shell    : Useful, not Unix-like shell commands.

uart:~$ 

Preparing for Measurements

Before making measurements ensure the DAC channels are set to the desired voltages and the TIA gain is set appropriately for your measurement. Refer to Serial Commands for a list of available commands to configure the device.

Warning

By default, devie is configured at startup as follows:

• DAC Channel 0: 0V
• DAC Channel 1: 1.25V
• TIA Gain: 1 KOhm

Important

It should be noted that the voltage accross the electrodes is determined by the difference between the two DAC channels. DAC Channel 1 is the voltage applied to the non-inverting input of the TIA. This is what enables the TIA to measure both, positve and negative going currents.

DAC Channel 0 is the voltage applied to bias the the load. This can be configured to be higher or lower than the voltage on DAC Channel 1 - enabling both, negative and positive bias.

The voltage accross the electrodes is given by: V_electrodes = V_DAC1 - V_DAC0 For example, if DAC Channel 1 is set to 1.25V and DAC Channel 0 is set to 0V, the voltage accross the electrodes is +1.25

It is important to know that the maximum output of each DAC channel is 3.3V.

Once configured, connect the DB-9 electrode lead plug to device under test. The pinout of the DB-9 connector is as follows:

Wire Color	Description
Red	TIA Input
Blue	Bias Voltage Output
Green	Bias Voltage Output (Shorted to Blue)

Making Measurements

Connect the red wire to the electrode where current is expected to flow into the TIA (anode). Connect the blue/green wire to the electrode where current is expected to flow out of the TIA (cathode). The blue and green wires are shorted together on the DB-9 connector, so either can be used.

When configured, the device can now be used to make current measurements. The ADC can be read using the adc read command. The output of this command is the converted current measurement appropriate units based on the configured TIA gain (uA, nA and pA).

uart:~$ amux mode 1
Analog multiplexer set to mode 1: A0=0, A1=1, resistor: 100k

uart:~$ dac write 0 0
DAC Channel 0 set to 0.000 V (10-bit code: 0)
Current DAC configuration: Channel 0: 0.000 V, Channel 1: 1.250 V

uart:~$ dac write 1 1.25
DAC Channel 1 set to 1.250 V (10-bit code: 387)
Current DAC configuration: Channel 0: 0.000 V, Channel 1: 1.250 V

uart:~$ adc read
Current: -2308.199 nA
[00:29:03.344,726] <inf> main: ADC Code: 0x1427
[00:29:03.344,757] <inf> main: ADC Voltage: 1.587 V

// Reduce Impedance from 2M to 1M

uart:~$ adc read
Current: -2853.904 nA
[00:30:53.839,843] <inf> main: ADC Code: 0x152A
[00:30:53.839,874] <inf> main: ADC Voltage: 1.667 V

// NOTE: The 500nA increase in current due to the 
//       decrease in impedance. 
//       This is within the expected difference 
//       between the measurements.

uart:~$ 

Warning

This unit is not precision-calibrated. Expect gain error (±30%) and offset errors, increasing at higher gains and in sub-nA/pA ranges. Use for relative trends rather than absolute measurements.

Note

TODO

• Improve Device Calibration
• Add user configurable gain settings in the Shell Interface
• Use NVR to store user settings across power cycles

Serial Commands

DAC Commands

• dac write <channel> <value>: Set the DAC output for the specified channel (0 or 1) to the given value (0-3.3V).

• dac read: Read the configured DAC voltages for both channels.

ADC Commands

• adc read: Read the current value from the ADC.

TIA Gain Commands

• amux mode <mode>: Set the TIA gain mode. Available modes are:
  • 0 for a 10 MOhm gain
  • 1 for a 100 KOhm gain
  • 2 for a 1 KOhm gain