The Circuit Design Blog

Infrared Thermopile Detector Amplifier

This amplifier is designed for a thermopile infrared detector such as an S60M from Dexter Research. This detector output is 101mV/mW with a source resistance of 90k. A CMOS autozero OPA335 is used to minimize offset and drift as well as 1/f noise. The detector response time is 12ms so the amplifier frequency response  is rolled off to minimize its total output noise.
R3 & C2 add an additional pole to the amplifier’s rolloff; this improves the total output noise compared to the conventional R2 & C1 feedback filter. This improvement is evident in the noise plot below. U1 output is unipolar, so C2 can be a good tantalum capacitor.

The OPA335 output stage can be pulled- down to zero by an external  negative supply V-, even when operated on a single +5V power supply.

By not operating the op amp at the conventional “Vcc/2” bias point, the output is capable of full  rail- to- rail out-put, increasing the detector amplifier’s dynamic range. (Circuit created by Neil P. Albaugh,  TI – Tucson)

Online Simulation of the Infrared Thermopile Detector Amplifier Circuit

The great feature of the TINA circuit simulator that you can analyze this circuit immediately with TINACloud the online version of TINA. Of course you can also run this circuit in the off-line version of TINA.

Click the link below to invoke TINACloud and analyze the circuit.

https://www.tinacloud.com/tinademo/tina.php?c=54b3ae1ebcf37%3A415378

You can send this link to any TINACloud customers and they can immediatelly load it by a single click and then run using TINACloud.

Michael Koltai
www.tina.com

Synchronous Modulator and Demodulator

Recovering small signals in the presence of noise is the application most suited to the synchronous modulator and demodulator technique. If a DC voltage or low-frequency AC signal is modulated by a higher frequency carrier (also known as “reference”) the input signal is converted to an AC signal – the carrier frequency sidebands. Demodulating the signal uses a similar technique (also known as “phase sensitive detector”) resulting in the recovered sidebands.

A low- pass filter is used to remove carrier artifacts and out-of-band noise.

Random noise generated in the amplifier integrates to zero if a very long integration time (a very low freq LPF) is used.

It is entirely practical to recover a small signal buried in 40dB of random (white) noise with a synchronous detector. This is sometimes also called a lock -in amplifier”.

The sync modulation/demodulation switches SW1 & SW2 are usually good analog switches. To prevent DC offset, it is important to maintain a precise 50% duty
cycle in the reference generator and switches. (Created by  Neil P. Albaugh,  TI – Tucson)

Synchronous Modulator and Demodulator circuit

Online  Simulation of the Synchronous Modulator and Demodulator Circuit

The great feature of the TINA circuit simulator that you can analyze this circuit immediately with TINACloud the online version of TINA. Of course you can also run this circuit in the off-line version of TINA.

Click the link below to invoke TINACloud and analyze the circuit.

https://www.tinacloud.com/tinademo/tina.php?c=54bd18289bcf0%3A724605

You can send this link to any TINACloud customers and they can immediatelly load it by a single click and then run using TINACloud.

Michael Koltai
www.tina.com

Single Supply Absolute Value Amplifier

This absolute value amplifier has a gain of 0.5V/V and has an input range of within a few mV of -5V to +5V. A buffer amplifier should be used on the output to prevent loading. For a faster amplifier, use an OPA354.

The rail- to- rail input and output characteristics of these CMOS op amps allow them to swing very close to their supply rails– +5V and ground. By using both an inverting and noninverting amplifier to swing only positive (due to their not being capable of swinging below ground) , each op amp acts like a perfect rectifier. Summing their outputs together in R3 & R4 combines them into an absolute value of the amplifier’s input voltage. (Circuit created by Neil P. Albaugh, TI-Tucson)

Single Supply Absolute Value Amplifier circuit:

The great feature of TINA that you can analyze this circuit immediately with TINACloud the online version of TINA. Click the link below to invoke TINACloud and analyze the circuit.

https://www.tinacloud.com/tinademo/tina.php?c=54c657263d83e%3A203818

You can send this link to any TINACloud customer an they can immediatelly load by a single click and then run using TINACloud.

Michael Koltai
www.tina.com