Low Noise Small signal Amplifier design question

[HLong]

I am looking for an application which can be used to compare the small input signal (0 to 10mV) and drive/trigger a 115VAC relay. The requirement is that the relay is off from 0 to 4mV. Between 5 and 10mV, it will trigger the relay. After the relay is closed, it will open again when the input is 4mV less than the signal previously set. Which OpAmp/Comparator or product line you have would be best for this application? What's your approach in designing such circuit? All response would greatly be appreciated. Thank you in advance.

P.S. Believe it this is used to replace the vacuum amplifier circuits in one of our application.

 

[VisiGoth]

Analog Devices publishes on this topic.
The major manfacturers should have nice app notes.

I would guess if you wanted to build your own instead of buying a solution you could find a nice op-amp and a pecision voltage source. Then use precision resistor to divide down and use positive feedback to do the hysteresis. I am assuming there is no loading problem and no problem in providing a signal inptu back into your signal source.

If your circuit was designed with a set point of 4.5 mV then that would be the definition of your "signal previously set". You would then create a hysteresis value offset of 4 mV, making the reset value 0.5 mV. That could be a little touch, especially with a relay clicking.

If somehow you need to remember the highest value recorded during a "set" interval you will need some sort of memory device. Analog memory peak hold devices have pretty much disapeared from use so you might want to go straight to an A/D converter with some sort of control mechanism.

 

[itsmoked]

If you are into building your own, an 8-pin PIC with an A2D would allow you to do any desired filtering and hysteresis both timed and direct.

 

[ScottyUK]

Burr-Brown make a good line of precision op-amps which will bring your signal up to something more manageable. Is the signal source single-ended or floating? You might want to consider an intrumentation amp or diff. amp to keep CM noise rejection high if it is floating.

Once you have a reasonable signal any of the standard comparator ICs should be suitable.


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One day my ship will come in.
But with my luck, I'll be at the airport!

 

[bill318]

I think I would amplify the signal by about 10 with an op-amp and then apply it to the comparator circuit with hysteresis as mentioned above. All could be done in a dual op-amp package including a bit of low pass filtering on the gain stage. You could then use the output of the comparator to drive an opto-triac which in turn would drive your AC relay. Place an MOV across the output of the triac to prevent the relay field collapse from triggering the triac. Otherwise it can make an awful sounding oscillator.

-Bill-

 

[Yashu]

I agree with itsmoked.

With the price and small size of microcontrollers these days, any low bandwidth, non-noise critical analog processing is formidably challenged by a sampled digital implementation.

A hi-gain amp with 8 pin uC cannot necessarily be ruled out for this app, apart from price.. I would think.