LTC6404-1 maximum output swing

Hi,

What is the maximum output swing I can get without clipping with a 3.3V / GND asymetrical supply ? I found the datasheet confusing on this point.

I read on datasheet page 4 that minimum V_OUTCMR is 1.1V. Does it mean that I will get clipping if (V_OUT+ + V_OUT-) / 2 < 1.1V ?

You also specify on page 4 "Output Voltage Low, Either Output Pin" (see figure below). Its value is 140mV at 3V 5mA for instance. What does this mean ? Does it mean that I won't get clipping if OUT+ and OUT- single ended voltage are above 140mV ? Is this this characterisitc I should look to evaluate the maximum voltage swing of the op amp ?

To give you some context, I want to use the LTC6404 to drive a 1.8V differential ADC with an input span of 2Vpp, and a common-mode voltage of 0.95V. Currently, the LTC6404 use a 3.3V / -1.8V supply to be sure there is no clipping around ground. Each output of the opamp is swinging between 0.45V and 1.45V, which makes me think there is no clipping, because 0.45V is above 140mV. But, I think that the -1.8V rail is not necessary in the end and that I won't get clipping if I use ground instead. I just need you to clarify the datasheet to make sure of it.

Thank you,

Maxime 

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  • 0
    •  Analog Employees 
    on May 24, 2019 7:54 AM

    Hi Maxime.

    Thank you for your questions. Please see our explanation below:

    What is the maximum output swing I can get without clipping with a 3.3V / GND asymetrical supply ? I found the datasheet confusing on this point.

    Since the datasheet only specifies parameters for VS = 3V and 5V, where VS = V+ - V-, and the difference of your supply voltages is 3.3V, you may refer to the specifications for VS = 3V to approximate the maximum output voltage swing.

    I read on datasheet page 4 that minimum V_OUTCMR is 1.1V. Does it mean that I will get clipping if (V_OUT+ + V_OUT-) / 2 < 1.1V ?

    V_OUTCMR is the output DC or bias voltage, and will depend on your input to the VOCM pin and on VS. Again, the V_OUTCMR range given in the datasheet is for VS = 3V and 5V only.

    You also specify on page 4 "Output Voltage Low, Either Output Pin" (see figure below). Its value is 140mV at 3V 5mA for instance. What does this mean ? Does it mean that I won't get clipping if OUT+ and OUT- single ended voltage are above 140mV ? Is this this characterisitc I should look to evaluate the maximum voltage swing of the op amp ?

    The "Output Voltage Low, Either Output Pin" parameter is the output low level headroom voltage. See note 10 in the succeeding pages of the datasheet. You will notice that this headroom voltage increases as the current load increases. For VS = 3V and IL = 5mA, for instance, the minimum possible VOH would be V+ - 600mV and for VS = 3V and IL = -5mA, the maximum possible VOL will be V- + 260mV. The resulting values will define the minimum possible output voltage swing. Using the typical values will result in a higher VOH and lower VOL, hence larger output voltage swing.

    To give you some context, I want to use the LTC6404 to drive a 1.8V differential ADC with an input span of 2Vpp, and a common-mode voltage of 0.95V. Currently, the LTC6404 use a 3.3V / -1.8V supply to be sure there is no clipping around ground. Each output of the opamp is swinging between 0.45V and 1.45V, which makes me think there is no clipping, because 0.45V is above 140mV. But, I think that the -1.8V rail is not necessary in the end and that I won't get clipping if I use ground instead. I just need you to clarify the datasheet to make sure of it.

    Please give us the part number of the ADC you are using so we can recommend a better setup for you. Better yet, send us your schematic so we can evaluate further. We also want to know your VIN specs so we can confirm if clipping occurs in your setup.

    Regards,
    Shine

  • Hi,

    The ADC reference is ADS5294. It works at a common-mode of 0.95V. Therefore I want to set the LTC6404 VOCM pin to 0.95V too, but it is below its minimum value which is 1.1V. What will happens?

    The simulation seems OK with that. I'a attaching the simulation file, which will gives you my VIN spec too. 

  • 0
    •  Analog Employees 
    on May 27, 2019 6:19 AM in reply to MPuech657

    Hi Maxime.

    The 1.1V minimum VOCM specification in the datasheet is for the ff. conditions: VS = 3V, V+ = 3V, and V- = 0V. In page 16 of the datasheet, VOCM is defined as the average of the two output voltages OUT+ and OUT-. If VOCM pin is left open, it is the midpoint of the V+ and V- pins. So if your V+ = 3.3V, you need to make V- more negative or around -1.4V to get a VOCM of 0.95V. But you can also connect an external supply capable of driving the input impedance presented by the VOCM pin, like what you did in your schematic.

    Your schematic seems OK but if you want to prevent clipping and get an output swing of 1.8Vp-p, you should consider making V- more negative to account for the headroom voltage.

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  • 0
    •  Analog Employees 
    on May 27, 2019 6:19 AM in reply to MPuech657

    Hi Maxime.

    The 1.1V minimum VOCM specification in the datasheet is for the ff. conditions: VS = 3V, V+ = 3V, and V- = 0V. In page 16 of the datasheet, VOCM is defined as the average of the two output voltages OUT+ and OUT-. If VOCM pin is left open, it is the midpoint of the V+ and V- pins. So if your V+ = 3.3V, you need to make V- more negative or around -1.4V to get a VOCM of 0.95V. But you can also connect an external supply capable of driving the input impedance presented by the VOCM pin, like what you did in your schematic.

    Your schematic seems OK but if you want to prevent clipping and get an output swing of 1.8Vp-p, you should consider making V- more negative to account for the headroom voltage.

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