AD9850 questions

Q 

We design and prototype a signal generator. One of the requirements is precise
amplitude control. We are considering a design based on AD9850 followed by
AD8370. We propose to replace the Rset resistor on pin 12 of the AD9850 by a
mosfet constant current source, similar to app note AN-423 except that the
mosfet will be driven from an op-amp and DAC. I have some questions about
controlling the AD9850 output in this way:

1. Is the AD9850 DAC output current directly related to the current sourced by
pin 12? if so, what is the relationship?

2. What is the minimum voltage we can expect on pin 12? We need to know this so
that we can ensure that the constant current source does not saturate.

3. What is the range of current control we can use? The AD9850 data sheet shows
output current range of 5 to 20mA. Can we use control to below 5mA? if so, what
is a reasonable lower limit?

A 

1. Yes. DAC’s External RSET Connection. This resistor value sets the DAC
full-scale output current. For normal applications (FS IOUT = 10 mA), the value
for RSET is 3.9k connected to ground. The RSET/IOUT relationship is IOUT =
32(1.248 V/RSET).
I have attached a circuit diagram showing how the Rset is arranged to produce
the DAC full scale current at the DAC output.

2. Use the formulas above to calculate the voltages on pin 12.

3. We did not fully characterize the device below 5mA. As you go down in full
scale current, the DAC linear could start to distort. We’ve had customers work
down to 1mA full scale current with no complaints. Not sure how much below 1mA
you can go.

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