Transimpedance amplifier (TIA) is a current to voltage converter, it is useful for the requirement to convert the low-level current of a sensor to a voltage. The TIA can be used to amplify the current output of Photodiode, photo multiplier tubes/PMT, Avalanche photodiode/APD, laser power meter, conductivity, tunneling current/STM and other types of sensors to a usable voltage. In its simplest form a TIA has just a large valued feedback resistor, R_f. The gain of the amplifer is set by this resistor.

The performance, in terms of response or bandwidth, in terms of peaking or overshoot, and in terms of noise or SNR, is an extremely complicated, nonlinear, and highly interacting function of:

• the feedback resistor
• the source capacitance
• the desire bandwidth, gain factor and noise level /SNR
• the selection of op amp (Voltage/Current noise, input capacitance, GBP, offset)
• the selection of component
• the PCB design

Some useful information: 

1.  Jerald Graeme, Photodiode Amplifiers—Op Amp Solutions
2.  Bob Pease, Waht’s all this transimpedance amplifer stuff, Anyhow
3.  Application Note from AD/Linear, TI, etc.

Here are two examples developed in our lab, to pushing the limits of performance for our home-built setup.

Example 1, Special TIA design for home-built high performance CE-C4D:

Example 2, Special TIA design for home-built high performance PHI:

Update time: 2021-11-20

Laser power meters are an essential equipment for anyone working with lasers. It will be capable of measuring powers from 10s of nW to 10s of mW. To make a precision power meter, two issues need to be concerned carefully. First, the large dynamic range of power is the primary challenge to designing the instrument. Second, the photo sensitivity (A/W) of photodiode is dependent on wavelength,  so a tight calibration for full spectral response is another challenge.

Home-built laser power meters, 1st version

Details:

• Open source Arduino UNO, C language
• 16 bit ADC with ADS1115, I2C
• 128 x 64 OLED display, SH1106 , I2C
• Silicon photodiode (300-1000 nm), diameter 5 mm
• Photiode TIA design

Circuit design and performance test

Update time: 2021-11-19

In CE, ions are separated based on their mass to charge ratio (electrophoretic mobility) upon an applied electrical field. The separation mechanism only depends on the BGE composition, applied electrical field and intrinsic properties of the ions.

In general a CE system is composed of three major parts. These include (i) the separation capillary, (ii) the high voltage power supply (HVPS)–this is the driving element common in all separations and (iii) a detector with appropriate data acquisition (DAQ) supporting electronics. This makes CE much more do-it-yourself (DIY) friendly, either with conventional capillaries or on microchip format.

A reliable HVPS is crucial to perform both separations and electrokinetic injection. The separation in terms of repeatability of the migration times can to a large extent depend on the HVPS quality. Several well-established companies sell both the ready to use HVPS and HVPS modules for the researchers to build their own instrumental parts. In particular EMCO, Spellman and Dongwen/China. Here, we try to build our own HVPS setup.

Typically, HVPS can apply up to ±30 kV and ±300 μA if designed for standard CE. Note that low power HVPS are sufficient for CE with currents in the microampere range.

Home-built HVPS for CE-C4D

Key feature:

• High Voltage output (typically 0 – ±15 kV, 30 kV available)
• Positive or negative polarity
• Output Voltage controlling by external voltage
• Output Voltage monitor
• Output Current monitor
• Overcurrent protection
• Compact design
• User-friendly

Circuit design

Performance test

Update time: 2021-11-18

Lock-in amplifiers (LIA) are capable of measuring the amplitude and the phase of a signal relative to a defined reference signal, even if the signal is entirely buried in noise, it is vital important for extreme weak signal (e.g. 10 nV) detection. The lock-in detection technique is described both in the time and in the frequency domain.

It uses the knowledge about a signal’s time dependence to extract it from a noisy background, this method is termed demodulation or phase sensitive detection. A lock-in amplifier performs a multiplication of its input with a reference signal, also sometimes called  heterodyne/homodyne detection, and then applies an adjustable low-pass filter to the result.  LIA looks like a extreme sharp bandpass filter, the Q value up to 10^6, so it is easy to isolate the signal at the frequency of interest from all other frequency components.

Home-built softwave digital lock-in amplifier
Frequency range: below 20 kHz (dependent on the speed of ADC )
Linear range: 1 μV to 10 V;  Noise level: 0.6 μV

Home-built Analog lock-in amplifier module

Frequency range: 300 kHz to 40 MHz
Linear range: 10 μV to 300mV;  Noise level: 4 μV
(This range will be extended by extra gain or attenuation)

High sensitivity CE-C4D detection with home-built Analog lock-in amplifier module

eDAQ vs Home-built instrument

Update time: 2021-11-18