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DAC8563: Output (vout) filtering for optimal DC output performance

matthew
Prodigy 100 points
Part Number: DAC8563
Other Parts Discussed in Thread: LM3409,

Hello,

I am using DAC8563 to set the IADJ pin on LED driver LM3409. This is a DC value, it is set once via software and stays the same value. VREF is 1.25V, I'd like to be able to use the full output range of 0V - 1.24V to the LM3409. 

I wanted to add a simple RC filter to the output to smooth things out. However I found 0.1 uF capacitance with 1 ohm series resistance will make the output ring. I think the datasheet confirmed that - but this isn't clear to me:

8.3.1.2 Output Amplifier The output buffer amplifier is capable of generating rail-to-rail voltages on its output, giving a maximum output range of 0 V to AVDD. It is capable of driving a load of 2 kΩ in parallel with 3 nF to GND. The typical slew rate is 0.75 V/µs, with a typical full-scale settling time of 14 µs as shown in Figure 31, Figure 32, Figure 75 and Figure 76.

Is this actually recommending NO output capacitance for best performance? And no more than 2 kohm to ground?

What about adding output resistance to ground (1K dac suckers) so that some output current is always flowing - good or bad idea?

I see on the TI's evaluation boards 1.02K series resistance is the only filtering added, no capacitance. But I think for these applications the bandwidth is important.

Looking forward to getting TI's expertise on optimal DC output performance with DAC8563.

  • +1
    TI__Guru 53183 points

    Hi Matthew,

    The issue here is not so much the load resistance to ground, but rather capacitive load stability due ot the 100nF cap load.  1Ω is not a sufficient RISO to prevent stability issues.  While we do not have a output amplifier SPICE model for this device, we can recommend >100Ω to improve stability.  Could you try ~200Ω with 100nF for your RC? If you need a higher cutoff frequency, I recommend reducing the capacitor before reducing the resistance.