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OPA2170: noise source with low voltage negative supply

Part Number: OPA2170
Other Parts Discussed in Thread: TLV9062, OPA2990

Hi there,

My customer is using OPA2170 in an inverting structure as below. The power supply is GND and -2.8V(not -2.5V in the picture). The circuit works, but there is 10mV pk-pk noise at output as below shows. if I designed this, I would at least add a Cpole to limit the bandwidth.

As the the design is fixed, I would like to analyze the source of the noise to reduce it. Here is the possible two,

1. The input impedance is big which covert Ibias into bigger noise;

2. Single supply reduce the CMRR.

Is there any other noise source for this output noise? Does using a 36V amp in a -2.7V low voltage make the noise bigger? Should I ask customer to change to 5V device like TLV9062?


  • Hi Jerry,

    There are a few possible causes to the source of the noise.

    1. High value resistors which contribute thermal noise. I would recommend eliminating R71 and R79 from the circuit. These resistors are typically only used with bipolar op amps to reduce the errors associated with the input bias current. Since the OPA2170 is a CMOS amplifier it is not necessary to add these resistors.

    2. Stability issue. Using high value resistors in the feedback will degrade the phase margin of the device because the large resistors create a pole with the input capacitance in the 1/Beta network. The cause the Rate of Closure (ROC) analysis to be >40 degrees. This stability issue can be correct by placing a capacitor in parallel with the feedback resistor (R66 and R73).

    3. Input Common Mode Voltage. The input common mode voltage is GND which is V+ in your design since the supplies are GND and -2.8V. The OPA2170 does not have an input common mode voltage that extends to the positive power supply. Using this device with the common mode voltage at V+ will potentially have significantly higher noise. Please see section 8.4.1 of the datasheet. While the table in this section does not list a noise spec, it does show degraded performance of other spec when used outside of the common mode range.

    I would recommend taking a look at using the OPA2990 if the customer wants to stay with a high voltage op amp. The OPA2990 is a rail to rail input device.

    The TLV9062 would also be an option if the customer wants to change to a low voltage op amp.

    It is very important to ensure that the op amp is stable with any device that is chosen. In addition, the noise can also be reduce by adding an RC filter on the output of the op amp.

    Thank you,

    Tim Claycomb