OpAmp Output Noise

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shge_4395051
Level 4
Level 4
25 sign-ins 25 replies posted First solution authored

Hello

I'm using a CY8C4246AZI-M445.  When configuring one of the opamps, I am receiving about 160mV off noise on the output when inputting 0V.  The configuration is non-inverting with a gain of 20.  I'm using external resistors and the output of the opamp connects to a pin.  I'm using an oscilloscope to view the input and output.  How can I solve this?  

thanks

Shawn

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1 Solution

Thank you for the advice.  I will most likely add an external rail to rail opamp.  They are only about $0.30 each.

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7 Replies
odissey1
Level 9
Level 9
First comment on KBA 1000 replies posted 750 replies posted

Please check o-scope grounding and probe integrity. Disconnect the scope Gnd from the AC power Gnd (use AC coupler or a cable with Gnd pin removed). Try another scope probe and scope channel.

Do you see less noise when testing  other pins of the mcu? What is the shape of noise, is it 60Hz AC? Post the screenshot.

It would be better if you post the schematic screenshot and project archive.

 

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I made a mistake in my post.  Zero volts DC is input but I'm reading 100mV DC on the output.  The Gain is set to 16 using external resistors.   The scope probe is grounded well.  

shge_4395051_1-1686005922252.png

 

 

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PSoC4 Opamps are not Rail-to-Rail type. That means neither input can be driven to the Gnd closer than approx. 100mV. This cut-off voltage depends on the Opamp driving mode, and is lowest (~20mV) in the LowPower mode. Once the input voltage goes below that "common mode" value, the Opamp goes into saturation and no longer functioning properly. The PSoC4 Opamp output also can't go all the way to the Gnd, typically stopping at +100mV. And that is what you, probably, observing.

The bottom line, it is not possible to use PSoC4 Opamps to amplify DC signals, which are close to Gnd.

On the other note, the values of the resistor divider 1M and 15M look too ambitious. The input impedance of the Opamp is high, but not that high. Usually, 10-100k values would be more appropriate with such low-end opamps. 

Please check this thread, showing several techniques of amplifying and measuring voltages, which go below the Gnd. 

Reading negative values ADC, PSoC5 LP

To measure signals in the 0-100mV range, I would consider external opamp with split +5V, -1V(-5V) power supply.

You can omit all that hurdle if using PSoC5 Delta-Sigma ADC instead of SAR ADC. It has built in buffer-amplifier and can measure voltages in 0-100mV range directly, without Opamp. 

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Thanks for your reply.  This makes sense.  I looked through the datasheet and couldn't find any specs for the non-rail to rail behavior.

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PSoC4 & 5 Opams are practically the same.  I will look into possible arrangement of the DiffAmp  and SAR_ADC in the differential mode.

Alternatively, to measure low voltages in +/-40mV or +/-80mV range you can use low-cost HX711 24-bit DeltaSigma-ADC module (~$1/pc). 

HX711: 24-bit Delta Sigma ADC interface for weight scale using PSoC

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Shown below a solution using PSoC4 Opamp in the Differential mode (Figure 1). When input voltage from signal generator VG1 varies from 0 to 100mV, the output (Vout) varies from 2.5V to 1.5V (Figure 2), while voltage difference between 2.5V reference VF1 and Vout varies from 0 to 1.5V, providing the Gain=15. Note that +/- inputs of the Opamp are at 0.17V, which is within CM range of the Opamp.

To measure voltage difference VF1-Vout, the SAR_ADC should be configured in Differential mode, with one of the inputs connected to the VF1, and another to the Vout. 

Drawbacks: (1) Accuracy of such scheme depends on the accuracy of the resistor divider: R1=R3, R2=R4. When this condition is met, the 0V input will produce 0V output, otherwise there will be some offset, which can be corrected by the ADC calibration or a trimpot. (2) One have to spare another Opamp to make buffered 2.5V reference (not shown). (3) The input impedance is rather low (R1=10k).

Figure 1. Schematic of the Differential Opamp amplifying input voltage 0-100mV into 0-1.5V output range.

DiffAmp_01a_SCM.png

Figure 2. Simulation of the above circuit using Tina-TI. Green - Vout, Brown  -  VF1 (2.5V Ref). 

DiffAmp_01a_DC.png 

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Thank you for the advice.  I will most likely add an external rail to rail opamp.  They are only about $0.30 each.

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