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DRV2700EVM: Does not work linearly in Single-Ended (DC Coupled) Input

Daniel Chopra
Prodigy 10 points

Part Number: DRV2700EVM

Tool/software:

Hi,

I want to vary the output voltage linearly (0V to +105V) in single ended DC input with set reference configuration, I have followed the steps mentioned in the DRV2700EVM user guide (SLOU403C) section 5.4.1, but the output voltage is non-linear with respect to the input DC voltage (0V to 5V) applied at pin TP7(IN+), with DC reference input at 2.5V applied on pin TP8(IN-).

Steps I followed:

  1. Removed jumpers JP6 and JP5
  2. Connected EN, GAIN0 and GAIN1 jumpers to PU as to enable the driver and config the gain to 40.7db
  3. Connected DC reference 2.5V to TP8(IN-) pin
  4. Connected a variable voltage source (0V to 5V) to TP7(IN+)
  5. Disconnected jumper JP_DC and powered the evaluation board with external power supply applied to J2 terminal
  6. Measured voltage with a multimeter between pins OUT+ and GND

Observations:

  1. As soon as powered up and with input 0V applied at TP7(IN+) the output voltage is 1.62V but not 0V
  2. from 0V to 1.6V input the output voltage is constant at 1.62V, no linear change
  3. from 1.7V to 3.5V input voltage the output seems to be jumping randomly from 6.25V to 103.2V, not following a constant gradient change
  4. from 3.6V to 5V input the output voltage is saturated to 103.2V, not reaching till 105V

Can you help me figure out where have I went wrong how to configure the DRV2700EVM to properly operate it linearly, is the amplifier after the boost converter a rail-to-rail amplifier or not?

  • 0
    TI__Genius 11152 points

    Hi Daniel,

    This is a fully differential amplifier, so when you say output voltage, what do you mean? Is it Vout = (Vout+) - (Vout-)? 
    If you are able to measure the difference between IN+ and IN-, then you should see that reflected on the output between Vout+ and Vout-.

    Daniel Chopra said:
    input 0V applied at TP7(IN+) the output voltage is 1.62V but not 0V

    This should produce a negative voltage, Vin+ = 0V and Vin- = 2.5V. Difference is -2.5V. Then multiply by the gain of 40dB (100x) -- differential output will saturate at ~ -103V. 

    There is a little headroom on the amplifier. The output voltage expectation should be +/-100V with a 105V boost setting. 

  • 0 in reply to Kelly Griffin
    Prodigy 10 points

    Hi Kelly,

    Thank you for your response, So when I meant Vout I'm referring to the voltage Vout+ with respect to ground as mentioned in the DRV2700EVM user guide, section 6.2 two terminal single ended output configuration as I want to operate my piezo element in the 100Vp range.

    So when I am measuring the output voltage between Vout+ and GND, I was expecting to see the output voltage change linearly corresponding to the difference between the IN+ and IN- .

    I was expecting a linear relation between the Vin and Vout as given in the DRV2700 datasheet "figure 6-13 output linearity" on page-8

    Can you clarify me on how to configure the inputs and outputs in order to obtain a linear relation as mention in the datasheet output linearity.

    Thanks

  • +1 in reply to Daniel Chopra
    TI__Genius 11152 points

    Hi Daniel,

    Thanks for the clarification. I looked back at the linearity plots, and they were created with the output being measured differentially (sorry, it is not clarified). The tricky part in that first image is the text "This output configuration allows for 2 outputs that are 180 degrees out of phase." For your case with a single ended output, your test method is correct. Let me explain a few things. 

    When you're driving 0V on the output, each output (+ and -) will be at VBST/2, producing 0V differentially. You see this in your original post for IN+ = 2.5V because you have IN- biased at 2.5V.

    As you increase the differential input voltage (Vin+ minus Vin-), then you will see two things happening with respect to GND. Vout+ will increase and Vout- will decrease. When (Vin+ minus Vin-) decreases, you will see the opposite.

    From 1.5V to 3.5V you should see a near linear change in Vout+ (or Vout-) with respect to ground, which your data reflects. This is because your gain is 40dB (100x). When the differential voltage is +/-1V, you will saturate the outputs in either direction.

    For example: 3.5V-2.5V = 1V, multiplied by 100 = 100V, which will be ~100V on Vout+ and ~0V on VOUT-.

    One last note here: Single ended measurements like what you are doing will show a little bit of noise ripple onto the VOUT pin from the VBST... This would cancel out in a differential measurement. You can see this on an oscilloscope. 

  • 0 in reply to Kelly Griffin
    Prodigy 10 points

    Hi Kelly,

    Thank you for the clarification now I understand it well on how to work with it, can you suggest a particular piezo driver in your line of products that is primarily design to control it linearly with a DAC (I mean as DC setpoint Vin) rather than with PWM or AC modulation because in single ended output the gain is too high 100, which is limiting my control signal to operate in 0 to 1V range rather than 0 to 3.3V or 0 to 5V  

  • 0 in reply to Daniel Chopra
    TI__Genius 11152 points

    I believe you are using the highest gain setting. Can you just turn the gain down to one of the lower settings (28.8 dB, 34.8 dB, 38.4 dB)? 
    I'm not that familiar with our DAC library of parts..there are a ton of them. Typically, I'm using a DAC integrated in an MCU versus a standalone DAC. Sorry, I don't have a suggestion here.