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Minimum input voltage on the Opamp component | Cypress Semiconductor

Minimum input voltage on the Opamp component

Summary: 10 Replies, Latest post by NealXu_2977301 on 06 Dec 2017 08:48 AM PST
Verified Answers: 3
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user_365962704's picture
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Hi all,

I'm not used to analog electronics, so i can not find what is the minimum voltage the opamp can "see" and amplify on the Opamp datasheet.

I know it's not a high end Opamp, but i would like to replace an external component with the internal Opamp.

The signal voltage i would like to amplify is about 5 mA, first on the Opamp, and then a second stage amplifier with the PGA. [very similar to this https://www.hackster.io/gul/pir-motion-sensing-d465d4

Any info will help me :D

 

Thanks in advance

Carlos

user_242978793's picture
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What Psoc device are you using? that makes a difference when it comes to spec's.

user_342122993's picture
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804 posts

Carlos,

I am not analog engineer, but this is my take: PSoC' OPAMPs are not rail-to-rail, and won't amplify 5mV (to GND) input signal. The signal has to be shifted from 0+5mV towards analog "GND" (Vref) : Vref+5mV. As an example, see this schematic of level-shifter:

http://www.edn.com/design/analog/4332077/Precision-level-shifter-has-exc...

Something similar can be done with PSoC OPAMP, but CMRR will be worse. In PIR detector that you mentioned (BTW, link is broken) the analog GND is also sitting at 1.24V, but inputs were decoupled with capacitors, which was OK to amplify AC signal from PIR detector.

On the other note, +5mV signal (to GND rail) is sufficient to be directly sampled by DelSig_ADC, which works down to GND rail, and then process digitized signal.

 

user_242978793's picture
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Here is a complete program for the CY8CKIT-048 you can get this direct from Cypress.com for $48.00 USD.. The kit is called CY8CKIT-048 PSoC® Analog Coprocessor Pioneer Kit. It has a lot of analog devices you can use to developed your own programs.

user_242978793's picture
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Here is the complete schematic.

user_365962704's picture
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@bobgoar

Hi, thanks for embed the image, that is the shematic i'm trying to replicate in the CY8CKIT-059 (5LP device) i have, i can not use the Analog Coprocessor because i will need use the USBFS present on the 5LP part.

@odissey1:

Yes, i'm pretty new to analog electronics, it's a whole world by its own, i was adding DC offset like the schematic above, making a Vref with the VDAC, buffering it with a follower Opamp and then "sitting" the sensor signal there, but i was not adding all the Bias circuit, will do that later this week.

If you still want to see the link is this https://www.hackster.io/gul/pir-motion-sensing-d465d4 same link that bobgoar posted :D

 

Thanks for the answers, it seems like yes i can amplify a 5mVpp signal with the internal Opamp.

Carlos

user_242978793's picture
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Just take the program I sent you and change the device to 5lp and see if it complies you may need to change some of the components but the program will flag them.

user_342122993's picture
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Carlos,
Regarding the OPAMP biasing, take a look on this YouTube recent video
https://youtu.be/FtdJ4e973bk

user_365962704's picture
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Great video odissey1, thanks for the link, i will check it a couple times more to try fully understand it. Also found some "guides" from TI of how to understand Opamp datasheets.

Most surely the Analog Coprocessor Opamps will have better specs than the 5LP ones, will check that too.

Carlos

user_365962704's picture
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Hi, i did a simple test (schematic below), but if i replace the 56 ohms resistors with 1k or 10k the OpAmp gets saturated, even when both resistors are the same value. Then if i place both resistors with 56 ohms i got a 2V signal, i were expecting a signal of about ~1 V.

A capture of the oscilloscope below, blue signal is the output of the WaveDac, green signal is the OpAmp output signal:

And the project attached, i don't think theres something wrong but i may be configuring something in a wrong way.

Carlos

NealXu_2977301's picture
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Hi...i am a new user here. As per my knowledge PIR detector have the analog GND is also sitting at 1.24V, but inputs were decoupled with capacitors, which was OK to amplify AC signal from PIR detector. On the other note, +5mV signal is sufficient to be directly sampled by DelSig_ADC.

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