Multi-Channel Programmable Gain Transimpedance Amplifiers from MAZeT

Transimpedance amplifiers, or TIAs, are frequently used in industrial measurement technology to convert and amplify very small currents in opto-electronic sensors. The transmission function of a TIA is based on Ohm's law, whereby the resistor required for the voltage-to-current conversion is placed in the feedback path of an op-amp. The resistance value therefore determines the TIA's amplification factor. The advantage of this circuit design over simply converting the current using a passive resistor is that the voltage on the sensor input can be matched to the given reference voltage. If the current sensor on the input side (such as a photodiode) is connected between the input of the transimpedance amplifier (inverting input of the operational amplifier) and the reference voltage (non-inverting input of the operational amplifier), the current sensor is ideally operating in a short circuit, i.e., the measurement only reads the current and is independent of the voltage. Additionally, this circuit design increases the signal bandwidth by compensating the sensor capacitance and also minimizes noise.

At its output, the TIA supplies a voltage that can be used in an ADC/microcontroller or in other analog components within the system. The electronic parts of a transimpedance amplifier need to be dimensioned to match the amplifier?s input specifications (input capacitance, sensor current, converter time, etc.) and their bandwidths, as well as taking into consideration interference values, tolerances and drift. Because of the wide range of uses for sensor current-to-voltage conversion and the many different parameters involved, the electronics market only offers very few standardized components for this purpose and most of these are for high-volume applications with standard configurations and low bandwidths.

MAZeT's range of MTI sensor amplifiers has had a new module added - the MTI08. The MTI range consists mostly of multi-channel transimpedance amplifiers. The transimpedance, i.e., the amplification and frequency, can be programmed over eight stages via the component?s pins. The amplifiers have high linearity and low noise, operate at a voltage of 3?5 V and are available as dies or in a QSOP housing.

All MTI components feature a power-down mode and can compensate large and small input capacitances (typically 5 or 50 pF). The monolithic integration of multiple channels into one substrate means the evenness of the amplification channels and the temperature coefficient of the transmission function are very even. The design is very compact, which means the components are cheap and easy to integrate.

Depending on the selected input capacitance, the bandwidth of the MTI components is between several kHz and a few MHz. Photo currents ranging from single-digit nA up to µA are converted into an output voltage of Vout = Vref - R*Iin (Vref is the supplied reference voltage).

Due to their large bandwidth, the MTI transimpedance amplifiers are particularly suitable for multi-sensor applications with low sensor currents. Typically, these would be opto arrays/rows (such as in the UV, VIS, NIR or IR spectrums) based on PIN, CMOS or GaAs/InGa technologies. This includes industrial sensor applications (light grids, measurement of lengths, fluid levels, positions, thicknesses and distances), surface analysis systems (color, shine, coarseness), temperature measurement (thermal imaging, combustion monitoring), other detectors for alpha, beta and gamma rays, x-rays and ions, as well as applications for measuring, regulating and controlling light and laser sources.

While the modules from the MTI04 series provide four amplifier channels that all use the same pin-programmed amplifier stage, the MTI08 provides eight channels that can be individually programmed via an SPI interface, with eight amplification stages ranging from 200 kOhm to 25.6 MOhm. The lowest input for the MTI08 is 20 nA using the highest amplification stage, with a signal frequency of 19 kHz / 22 kHz. The lowest stage is at 387 kHz / 563 kHz with an input current of 2.56 µA.

With both modules, the developer can either dynamically adjust the amplification of all the channels together to match the process conditions, or alternatively program each channel individually in INIT mode. In contrast to the MTI04, the MTI08 additionally features a multiplexer on the output.