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OPA335: Maximum output Voltage swing if G=1

Gerhard van Rensburg
Intellectual 400 points
Part Number: OPA335
Other Parts Discussed in Thread: TLV333, TLV274

My understanding: 

Maximum output Voltage swing if G=1 and 0V to +3.3V supply; (OPA335 has Common-Mode Voltage Range VCM (V–) – 0.1 (V+) – 1.5 V)

Would be -0.1 (Perhaps with negative supply) and +1.8V? (3.3V - 1.5V)

Unless one changes the gain to more than 1?

I use it as a buffer to ADC, in multiple channel DAQ, but only see 2.5V on some outputs, and correctly 3.3V on some, if the input reaches 3.3V

Perhaps the rail to rail to the layman is misleading.

So, If I change the part for one with VCM Common-mode voltage range (V–) – 0.1 (V+) + 0.1 V that should solve that problem?

Any help appreciated.

  • +1
    TI__Guru 51651 points

    Hi Gerhard,

    Gerhard van Rensburg said:
    If I change the part for one with VCM Common-mode voltage range (V–) – 0.1 (V+) + 0.1 V that should solve that problem?

    You mentioned that the op amp is using 3.3V as single supply rail, and you want to configure the op amp as buffer configuration with 2.5Vdc at the output. However, OPA335's input voltage range is specified from ~0V to +1.8V for 3.3V single supply rail, which is the dilemma.  

    Yes, you are able to resolve the input Vcm issues, if a selected op amp has wider input voltage range, say from  (V–) – 0.1 to (V+) + 0.1 V, namely Rail to Rail Input and Output (RRIO) op amp. 

    OPA335 is not Rail to Rail Input (RRI) op amp, but it is Rail to Rail Output op amp. So you would need to select an op amp with RRIO for the low voltage application (as an op amp buffer), this will solve the issue. 

    Since OPA335 is zero drift, low Vos, 2MHz BW, RRIO and other performance features that application may require, I sort it out the zero drift op amp selection, as shown in the table, also see the link below. We have a quite bit selection in the category.  

    https://www.ti.com/amplifier-circuit/op-amps/precision/products.html#480=1%3B2&2192=Zero%20Drift&1261max=3.6%3B5.5&1261min=1.6%3B6&sort=1130;asc&

    If you have other questions, please let me know. 

    Best,

    Raymond

  • +1 in reply to Raymond Zhang1
    TI__Guru 61800 points

    Adding to Raymond's answer, even though there are true rail-to-rail input common-mode op amps like the ones listed above that may swing to (or even beyond) its supply rails, NO op amp OUTPUT may swing all the way to its rail - see AOl  conditions shown in datasheets.  

  • 0 in reply to Raymond Zhang1
    Intellectual 400 points

    Raymond, thanks for the reply. I changed the op-amp to a TLV333 and the sensing now works great.

    ---------------------------------------------

    I have a different design that uses a TLV274IPWR, that is buffering and filtering a 1200 Baud signal, and I see 'noise' on the different stages.

    Would the same issue apply, in a 3V3 circuit, even though not used as a G=1 buffer?

    The individual stages have gain applied to them, so the outputs should swing to +3V3, but is the Common mode limitation still causing noise, as in crossover distortion?

    I will post this question under the TLV274 section as well.

    Any help appreciated.

  • 0 in reply to Gerhard van Rensburg
    TI__Guru 51651 points

    Hi Gerhard,

    TLV274IPWR is supported by the General Purpose op amp team. It looks like it is also RRO output amplifier, not RRI type. 

    Please submit the E2E inquiry and describe "noise" issues, and our team should resolve it very quickly. Thanks for the feedback!

    Best, 

    Raymond