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TLC7528 Digitally-Controlled State-Variable Filter

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Hi, I would like to build Digitally-Controlled State-Variable Filter. See on page 13 in http://www.ti.com/lit/ds/symlink/tlc7528.pdf. The problem is that data converter TLC7528 does not operate as shown in datasheet. Specifically formula "DAC equivalent resistance equals: (256)x(DAC ladder resistance)/(DAC digital code)" does not apply. Between pins Uref and Iou I get resistance only 10-40 kOhms. Do you have any experience with this circuit? 

Thanks, Jan. 

  • Hi Jan,

     

    Refer to Figure 11 of the TLC7528 datasheet for the architecture of the device ladder. The DAC ladder resistance is equal to the value of the divider resistor, R. The DAC ladder and the divider resistor form a symmetrical voltage divider which makes the voltage at the first node of the DAC ladder equal to REF/2.

    In voltage operation the OUT pin is a virtual GND, making the impedance from REF to GND = 2R or DAC ladder + divider resistor. 2R = 20kΩ as reflected on page 4 on the datasheet. This means that R = 10kΩ, making the DAC ladder resistance = 10kΩ.

       

    I took some values that you can look at.

     

    TLC7528 DAC Ladder Measurement

    REF =

    5

    V

    Measured DAC Ladder =

    10

    kΩ

    Code

    Max Eq. Res.

    Measured Eq. Res.

    Current OUT to GND

    256

    10.00

    kΩ

    9.43

    kΩ

    530.38

    µA

    128

    20.00

    kΩ

    18.71

    kΩ

    267.26

    µA

    64

    40.00

    kΩ

    37.25

    kΩ

    134.22

    µA

    32

    80.00

    kΩ

    76.53

    kΩ

    65.33

    µA

    16

    160.00

    kΩ

    152.58

    kΩ

    32.77

    µA

    8

    320.00

    kΩ

    304.14

    kΩ

    16.44

    µA

    4

    640.00

    kΩ

    694.44

    kΩ

    7.20

    µA

    2

    1280.00

    kΩ

    1366.12

    kΩ

    3.66

    µA

    1

    2560.00

    kΩ

    2673.80

    kΩ

    1.87

    µA

     

    We are also working on the preliminary stages of a TI Reference Design which uses the DAC8822 in order to construct a Digitally Controlled State-Variable Filter. This may be a few months away from completion, but check back in a couple of months if you are still interested in this application.

     

    Hopefully this cleared the confusion. Let me know if you need anything else from my part.

     

    > Eugenio Mejia

    Precision DAC Applications Engineer                 

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