There seem to be pesky speed bumps everywhere on the road, in driveways and parking lots. Although they vary in size, they are all equally unflattering. When you hit these speed bumps, you can choose to slow down to reduce wear and tear on the vehicle, or you can go back, but the best way is to go around.
    Yesterday, as I was slowing down through a pesky speed bump, I suddenly thought of my precision 16-bit R-2R DAC. It has a short-time pulse waveform interference problem in the middle scale (see Figure 1). I think that when you choose to use a DAC with a large short-time pulse waveform interference characteristic, you can add some anti-interference circuits at the DAC output to reduce the impact of interference. Two common DAC de-jamming circuits are simple low-pass filters (equivalent to a method of slowing down), and sampling/holding circuits (equivalent to "bypassing" interference). Both of these decontamination circuits can reduce the interference amplitude, or remove the interference energy.


Power supply system

Figure 1 Unwanted interference from a 16-bit DAC
    The simplest DAC de-jamming method is to use an R/C filter at the DAC amplifier output (VOUT) (bottom of Figure 2). This filter can reduce the amplitude of the interference, but increase the setup time.
    The curve at the top of Figure 2 is the signal for the DAC load DAC (LDAC) pin. Using DIN (data input) pins and CLK (clock pin), a data word can be loaded into the DAC in serial mode. Once the DAC has all the data, the rising edge of the LDAC pin loads the data word into the internal DAC register. This changes the DAC output voltage. The curve in the middle represents the measured DAC output intermediate scale analog interference. The bottom curve represents the analog signal measured using an R/C low-pass filter.

Power supply system
    Figure 2 R-2R DAC8881 (Vref = 5V, AVDD = 5V) middle scale transcoding
In general, this process is relatively smooth. As you increase (or decrease) the data code value, the output voltage rises (or falls). But at the mid-quarter and three-quarter ticks, the DAC interferes. The middle scale interferes the most.

    To determine the correct R/C ratio, first look at the interference time and then select a 3dB point for your filter, which is about 10 times lower than the interference frequency.
    For example, the interference time in Figure 2 is about 1 μsec, which is converted to 1 MHz interference time. From this estimation, we know that the R/C values in Figure 2 construct an 80 kHz low-pass filter. When selecting your R/C value, make sure that R is low enough to avoid load errors.
    This R/C filter solves the R-2R DAC interference problem, but it is not a "free lunch." As you can see in the bottom curve of Figure 2, the R/C filter extends the DAC's build time output signal.


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