0
\$\begingroup\$

I'm using a 8-bit DAC: (DAC5311IDCKR) to generate a 1.5V Amplitude sine wave with a 3.5VDC offset. I've put high-pass RC filter across it shown below to block DC so that it becomes AC.

RC Highpass I understand that the capacitor will act like a voltage source, and my SPICE simulator tells me that the cap's voltage becomes greater the source and (ideally) pass current back into the DAC. I'm concerned that this will either, damage the chip or not work at all because the chip doesn't allow currents in reverse.

  1. Is my concern valid?
  2. If so, when choosing a DAC what features should I look for on datasheets that tells me I can drive a load like this?
\$\endgroup\$
2
  • \$\begingroup\$ Does the DAC produce an output VOLTAGE (thus a cap is OK) or a CURRENT, which will need a DC_path? \$\endgroup\$ – analogsystemsrf May 21 '19 at 23:59
  • \$\begingroup\$ Is R2 >>R? ..... \$\endgroup\$ – Tony Stewart EE75 May 22 '19 at 0:19
0
\$\begingroup\$

1 uF is not like a voltage source, rather it is -j362 Ohms impedance at 440 Hz.

The DAC sees an impedance of 1k - j362 and your cap sees 3.5V + an apparent AC voltage drop from 1.5V. It does not exceed the applied voltage.

The important factor is to choose C and R load such that it is much greater than the DAC driver output impedance and C such that it’s impedance is much lower than R or visa versa.

An active filter may be a better choice so that lower currents pass thru the passive parts or a smaller C can be used and still provide a low output impedance required to drive your load.

The DAC output impedance at DC is 0.5 Ω.

Although R1 is not provided, I do not see any damage issues, but there are attenuation issues do possible back drive currents of 5mA during shutdown thru the CMOS driver that might trigger shoot thru currents during shutdown.

Re-examine your filter design for improvements with specs.

\$\endgroup\$
1
  • \$\begingroup\$ Oh, I see now. The DAC would just see a resistive + capacitive impedance, so we'd just treat that as an impedance. But, that still takes me back to my other question which was about the in cycle when the capacitor discharges and there is negative current flowing back into the DAC: puu.sh/DvAJH/093e1f6dfe.png (The current through the DAC-out is the blue waveform. Green is the DAC-out voltage.) How do we know whether a part can handle that current? \$\endgroup\$ – Recorren May 22 '19 at 6:04

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Not the answer you're looking for? Browse other questions tagged or ask your own question.