# amplification,how to amplify 50HZ, 300uV signal [closed]

i have to amplify 50hz frequency 300 micro volt signal, so please suggest any opamp that amplify low frequency signal and i have to amplify 300 micro volt to 3.3 volt hence, i need 10000 gain opamp hence which opamp is meet this criteria orany other modules are available?

• 10000 = 100 x 100 , lm 358 dual op amp - 100 gain keeps resistor values at a reasonable level Commented Mar 23, 2017 at 11:15

ln general Operation amplifiers act as a Low-pass filter which should allow 50Hz frequencies to be amplified fine by most Op-Amps (Unless you design a specific filter circuit with it). For the gain you should take note of the OL gain (which acts as the max gain) and the rain voltage (in other words you have to provide external voltage bigger than 4.7~ volts (which is 3.3+1.4 or so)

So I recommend checking some generic Op-Amps and seeing if they fit that critiria. I am pretty sure most of the OPAMPS should be fine.

Also note that the theoritical OL gain is at least 10^6 which is higher than your situation. Note that you should be using an Inverting/Non-Inverting Circuit for your OPAMP for the best results (and to control the output voltage)

Edit: A quick check shows that nearly all (if not all) Op Amps should work at low frequencies as they also work with AC. So Unless the OPAMP you choose has a problem with low frequencies or you construct a filter circuit nearly any OpAmp should be fine.

As suggections from the comments a low-offset voltage opamp would be best, Examples: http://www.st.com/en/amplifiers-and-comparators/low-offset-voltage-op-amps-lt1-mv.html?querycriteria=productId=LN1589

• The LM741 is an awful opamp to recommend.
– user103993
Commented Mar 23, 2017 at 10:39
• Is LMV358 module can amplify 50hz signal? Commented Mar 23, 2017 at 10:39
• @HayMan It is the most common and easiest one to get for most people Commented Mar 23, 2017 at 10:40
• @yogesha let me check. It should be but I can double check the datasheet for you Commented Mar 23, 2017 at 10:40
• It might be the most common but that doesn't change the fact it isn't a good choice at all. The output voltage swing is awful so you can't use it in a single supply configuration realistically.
– user103993
Commented Mar 23, 2017 at 10:42

If I was addressing this design requirement I would use an op-amp based design to produce the gain requirement. The op-amp you select should be one of a modern type that has very low input offset voltage down in the uV range or has a pair of NULLing pins to which you can attach a trimpot to adjust the input offset voltage to zero.

The second thing you want to do is to provide the gain in two stages of 100. Two stages will give a net gain of 10000 that you indicated is needed. Since your signal is an AC signal I would AC couple the input stage and the output of the two gain stages to remove any DC offset from either the input signal or accumulated in the two amplifiers.

My favorite general use opamp is the LT1638 and it should be a reasonable starting point for this type design as long as you AC couple in two stages like I mention above.

It may seem that 50Hz is low frequency, but if you want accuracy in the ~1% range you need an open-loop gain of at least 10^6 at 50Hz, implying a GBW product of 50MHz. Also if possible it would be best to AC couple the signal otherwise any small amount of input offset will be multiplied by the gain.

For such a large gain it's usually best to use multiple stages, for example two AC-coupled gain-of-100 amplifiers cascaded. With a gain of only 100, the GBW product can be reduced to only 0.5MHz so most (non-micropower) op-amps are suitable.

As far as power supplies and so on- since you've specifically asked for 3.3V it sounds like you might be planning to feed this into an ADC. This is a bit of a can of worms, because the input is presumed to be sinusoidal and centered about 0V and the ADC would typically like to see a signal between 0V and Vref, where Vref could be 3.3V. Negative input voltages may make the ADC very unhappy.

Lets examine the SNR achievable; here is DC_blocked topology, using opamp with 123dB DC gain, and 50MHz UGBW:

Examining the small plot, at 50Hz the Gain Margin is 127dB - 80dB = 37dB; with 40dB giving 1% gain accuracy, this single stage achieves 1.4% accuracy, which is about what you'll get with a couple of 1% resistors (1MegaOhm, and 100 Ohm, with 100uF in series with 100 Ohm, to remove the DC_offset problem).

Our upper F3dB is near 5,000Hertz, much higher than we need.

Look at top right in the screenshot, and notice SNR is 43.8dB at the FOI --- frequency of interest --- 50 Hz. However, I have deactivated the final stage of the signal chain, the Low Pass. Why is a final LPF useful? Here is why:

The chosen opamp, chosen for high DC gain and for high UGBW so the 50Hz has accurate gain, has a 7.5nanoVolt/rootHertz KT_thermal_Johnson_Boltzmann noise density. Must we spend more money for a low-noise OpAmp? Some opamps provide 1nanovolt/rootHertz noise density.

Lets evaluate that final stage, the RC Low Pass Filter, set at 100Hz F3dB. Notice the SNR rises --------- to 60dB, and 9.8 Effective Number Of Bits.