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OPA191: OPA191

Part Number: OPA191
Other Parts Discussed in Thread: ADS1015, OPA197, OPA397, OPA350, OPA333

Hi,

I am looking for the headroom voltage value of OPA191. How can I find this value in the datasheet? Is it possible to find it out? 

Thank you. 

  • Hi Emre,

    you will find it on page 8 and page 10 of datasheet under "voltage output swing from rail".

    As example: For a supply voltage of +/-10V and a 2k load the typical maximum positive output voltage is +9.8V and the typical mimimum negative output voltage is -9.8V.

    Kai

  • Hi kai,

    Thanks a lot for your reply. You solved my issue. So this means, "headroom voltage" is known as "voltage output from swing from rail". Got it. Best regards. 

  • Just to be clear, the Voltage Output Swing from Rail is the closest the output may get to rail under given load and supply voltage when the output transistors are fully trioded - a non-linear operation where output transistor becomes a resistor, Ron.  Under such condition the output stage loses most of its gain and thus open-loop gain, AOL, falls off a cliff - suddenly and quickly gets much, much lower. 

    Therefore, if you are interested in a LINEAR output voltage headroom of OPA191, where op amp maintain high AOL, you must look at the conditions of AOL and respective minimum specified limits for a given load and supply voltage - see below.

  • Hi Marek,

    Firstly thank you for your additional comment. I will use OPA191 as a voltage follower to read an ADC value. Circuit connection is like Sensor->OPA191->ADC". You talked about the transistor that inside the OPA191 right? I almost understand your comment but I couldn't figure it out. Could you give me one example for a condition or could you explain more? It will be more clear. To understand well. 

  • Hi Emre,

    can you show a schematic?

    Also, what is the signal voltage range the sensor gives out? And what ADC do you use?

    Kai

  • Hi kai,

    I am using ACS712 current sensor. And OPA197 as voltage follower. And ADS1015 to read ADC value. I didn't have a schematic yet so I can't share with you now. But entire system is as I mentioned up. 

    I also find something else while trying to simulate opa197. When I add a lower resistance value which is 120 ohm in the opa197's output. I see lower voltage value than my input voltage value at the output of opamp. But when I add 4.2MEG resistor, I see almost same voltage value at the input and output. 4.2MEG is also a resistance value between ADC's input pin and GND pin. I measured this value with multimeter. I this a good approach? My main purpose is "not losting ny of voltage value from my real value that coming from sensor"

    -120 ohm

    -4.2MEG

    Best Regards.

  • Hi Emre,

    the minimum supply voltage of OPA191 and OPA197 is 4.5V. So using a supply voltage of 3.3V is no good idea. Also, 120R presents a very high load to the OPAmp.

    You should choose a typical RRIO (input output rail-to-rail) OPAmp which can be supplied down to 3.3V. The OPA191 or OPA197 might not be the best choice. And increase the load resistance.

    Kai 

  • As Kai mentioned, OPA197 minimum supply voltage is 4.5V so it will not work well using 3.3V.  Also, under your conditions the output must source to the load over 27mA (3.25V/120ohm) and thus looking at the graph of Vout vs Iout, it is clear that under such heavy load the output needs almost a 1V headroom - see below.  This goes together with your simulation shown 2.4V output on 3.3V supply (~0.9V headroom).

    OPA397 would work on 3.3V BUT again its swing to positive rail under 27mA load is about 0.5V - see below.

    All in all, OPA350 may have the closest output voltage swing to positive rail under 27mA load - see below.

  • Hi kai,

    Firstly, sorry for late reply and thank you for your answer. I see now, it is wrong choice to use opa191 with 3.3V supply. Actually I was trying to model ADC's input pin. So I added 120 ohm resistance and 4.2MEG resistor. I measured 4.2MEG with a multimeter between the ADC's input pin and GND pin. So if 4.2MEG is really true then I have almost no voltage headroom at the output of the opamp. And this is good for me. I should change the opa191 because of 3.3V, by the way. I just want to know is my modelling true or what? 

    Best regards.

  • Hi Marek

    Firstly, sorry for late reply and thank you for your answer. Now it is clear and I understand what you are talking about. Thank you for the pictures. And thank you for suggestion of opa350 I will check it out. By the way I have a new question I wrote it down to "kai" . I hope kai will see it and I want to ask to you again; ""Actually I was trying to model ADC's input pin. So I added 120 ohm resistance and 4.2MEG resistor. I measured 4.2MEG with a multimeter between the ADC's input pin and GND pin. So if 4.2MEG is really true then I have almost no voltage headroom at the output of the opamp. And this is good for me. I should change the opa191 because of 3.3V, by the way. I just want to know is my modelling true or what? ""

    Best regards 

  • Hi Emre,

    what ADC do you intend to use? And how many times per second do you want to sample the input signal?

    Kai

  • Hi kai,

    I am intended to use  ADS1015 and I will get 4 sample in 1 second. How this effect my circuit ?

    Best regards

  • Hi Emre,

    the 120R resistor in your simulation wasn't totally wrong. But the right end is never short circuited when having this resistor between the OPAmp output and the ADS1015 input. According to figure 11 of datasheet of ADS1015 the right end of this 120R resistor would go to the input capacitances of ADS1015, which do not present any short to signal ground. So if you want to simulate the behaviour of OPAmp with this 120R resistor, the right end would have to be connected to these input capacitances, but not to signal ground.

    Section 9.2 of datasheet of ADS1015 shows a very nice example with the OPA333. Why not adopting it?

    Kai

     

  • Hi Kai,

    I saw the circuit that you said to me.

     

    In this circuit, R5 value is suggested as 100 ohm. But circuit is not ending at this point. Its going like "opa333->R5->AIN" how Can I simulate this connection? I dont want to think just 100 ohm, I actually want to know total resistance when measure from the output of OPAMP and ADC's ground. So what I am trying to understand is if I connected "SENSOR->OPA333->AINP" how much voltage will I have. I want to know deep about it.  And I am not sure that I my way is true or not.

    Best Regards.

  • Hi Emre,

    just add the common mode input impedance and differential input impedance of ADS1015 given in section 7.5 of datasheet of ADS1015. Also read section 8.3.2 of this datasheet. It will turn out that the 100R resistors and the input impedances of ADS1015 form a simple resistive voltage divider with a negligible damping.

    Kai

  • Hi Kai,

    Thank you for your response. now I understand the R5 value in the datasheet's example. But that opamp used as an amplifier because they are measuring current via shunt resistor. But I will not read current from a shunt resistor, I will use hall-effect resistor. And I will use opamp as a voltage folllower. And actually my main reason to add a resistor in front of the opamp is, adding a charge bucket circuit. As following picture;

    Bu while I am writing this, I realise that ADS10015 is not a SAR adc so not need to a driver circuit as charge bucket :) I am okey with that now thank a lot. But what if my adc will be an SAR ADC? How the below picture ,that I draw on it side , effect my opamp's output? 

    Best Regards.

  • Hi Emre,

    it is true that the ADS1015 is no SAR ADC, but there's enough high frequency switching of internal input capacitances in the ADS1015 that you will still profit from a charge kickback filter, as it is shown in section 9.2 of datasheet of ADS1015.

    But there's another reason why having a filter behind the OPA333. The OPA333 is a chopping OPAmp generating some high frequency switching noise. A low pass filter will eliminate this chopping noise and give better ADC readings.

    Kai

  • Hi Kai,

    it is true that the ADS1015 is no SAR ADC, but there's enough high frequency switching of internal input capacitances in the ADS1015 that you will still profit from a charge kickback filter, as it is shown in section 9.2 of datasheet of ADS1015.

    I understand this section. thanks a lot. 

    But there's another reason why having a filter behind the OPA333. The OPA333 is a chopping OPAmp generating some high frequency switching noise. A low pass filter will eliminate this chopping noise and give better ADC readings.

    And now I understand the other benefit of that RC circuit. 

    Thanks a lot for all of your messages. 

    Best Regards.

  • Happy to help Relaxed