AD7809: Protecting the analog outputs

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Q We are using an AD7809 which will drive a pen plotter which has an input impedance of 10Kohms. We are concerned about protecting the analog outputs against accidental short to ground. 1. What is the maximum output current? 2. The data sheet mentions a DC ouput impedance of 2 ohms, but the output load in the graphs is shown as 2 Kohms. 3. Also, the graph shows 4 ma. of sink current across 2 Kohms -which implies 8 volts at the output . However, that exceeds the supply voltage. Please explain. 4. How do you recommend protecting the outputs? A 1. The maximum output current to prevent damage to the device is not defined. Note 2 on page 6 of the datasheet says that the outputs may be shorted to any voltage between -0.3V and AVDD + 0.3V without damage PROVIDED the power dissipation of the package is not exceeded. With 8 outputs shorted to ground and the device operating at high ambient temperature, there is clearly a risk that the package won't be able to dissipate all the heat generated. There is no short-circuit current specification on the datasheet so it is difficult to assess how much power is generated by one/several outputs being shorted. We can guess some things about the output characteristics of the DAC's buffers by looking at figure 31 of the datasheet (Vdd = 3V). You can see that the full scale output voltage has a "knee" in the output characteristic when you draw more than 5mA. At his "knee" point the gradient changes from 2 Ohm output impedance around a zero current point, to about 40 Ohm output impedance as the buffer sources >4mA. If it continued at this rate it would mean approximately 75mA output current would be drawn when voltage was pulled down to ground. I think that this is probably pessimistic and the output will probably current limit before this point. Since each output is safe to short circuit, it is easy to do this test. Simply connect an external voltage source through a series resistor, set the DAC output up to full scale and the external voltage source to the same as the DAC output voltage; then I = 0. Then reduce steadily the external voltage source and measure the voltage drop at the output of the DAC and the current drawn. If you plot the points as in figure 30/31 you get the output characteristic of the DAC under your conditions of supply and RL. Or simply measure the short circuit current directly with an ammeter that has a full scale current of > 100mA. 2. The DC output impedance at low current levels is 2 Ohms, as shown in figures 30 and 31. Rout will change when higher current is sunk or sourced. All specs are guaranteed assuming a load resistor of no less than 2k Ohms. 3. The graphs of figure 30 and 31 are a bit confusing. They were obtained, I'm sure, in the way I described in point 1, i.e. using an external variable voltage source connected to the output resistance to control the output current, rather than using a ground terminated load. For RL = infinity measurement, a current source was probably used (infinite impedance). 4. You could use an external series resistor <<10kOhm to limit the current and power dissipation in the case that more than one output is shorted at once, the trade off is that you will get a gain error with a larger resistor, but the device will be better protected.

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AD7809: FAQ