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Problem with some hardware using OPA832 amplifier.

Other Parts Discussed in Thread: OPA832

Hi all.

We are one company to control public lighting.

Now, we have designed one hardware product with OPA832 amplifier in order to have 1-10V control for ballasts. The ballast drives the lamp. With 0V, the lamp is off. With 10V, the lamp is in 100% power.

I write you because hardware product requires OPA832 amplifier in order to amplifier by 2 the input voltage, so it is no necessary to include the gain resistors regarding OPA832 datasheet.

The power supply is 10.94 volts.

We have made many units and almost all works well, but I saw one or two (of 50 pieces) that output of OPA832 is 0V. First, it worked good, but some days after, the output did not work, it is always with 0V.

Is any problem with OPA832 in single-supply mode powering with 10.94V (absolute maximum rating is 12V)? 

I attach schematics.


Thanks in advance.

Francisco Jimenez.

  • Francisco,

    One item that catches my eyes is that you are showing a 1uF capacitor directly on the output of the amplifier. This is never recommended for a high speed amplifier. There are several reasons for this, but to get straight to the point, this can easily cause instability/oscillation of the amplifier. If you had placed a series resistor (at least 20 ohms with 50 ohms being better) directly on the output before the capacitor, then it should work fine. However, the resistor and the capacitor obviously forms an RC filter - good or bad.

    As a general note, ti is almost always preferred to have some sort of series resistor on the output no matter what it is driving - high impedance like an ADC input or not. The reason is to minimize parasitic capacitance and the impact on stability. The resistor should be placed as close as possible to the amplifier output.

    The only other thing that I see is it appears there is a diode on the + supply into the OPA832. If there is a glitch on the supply, it is possible that the diode may ultimately cause an over-voltage condition to occur until the voltage eventually bleeds away. This scenario may be OK if it is fed from a voltage regulator or LDO, but the schematic does not show this.

    Last possibility is an overvoltage on the + input pin. I am not sure where this signal comes from, but if the voltaeg could exceed the allowable input range, then this could be an issue. Adding some resistance in series could potentially help this.

    Cheers,

    Randy

  • Thanks Randy for your soon response.

    I will change and test with a fewer value por capacitor and locating a serie resistor.


    About other points you ask:

    a) Regarding diode, its voltage comes from one LDO regulator.

    b) With non-inverting input, its maximum voltage is 5V+0.5V. It is fed from one 5 Volt microcontroller.


    I will test and then I will report


    Thanks and best regards.


    Francisco.