Question
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?
Answer
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 <
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.