# AD9851: Best value for Rset

Document created by analog-archivist on Feb 23, 2016
Version 1Show Document

### Q

In your datasheet you show an AD9851 with both outputs connected to a
Minicircuits T1-1T transformer, the primary center-tap is grounded.

What value of Rset should be used in this configuration?

### A

Rset sets the full scale output current of AD9851. Output current can be set
from 5mA to 20mA with optimal performance at 10mA.

The value of Rset is related to full scale output current IOUT by Rset =
39.93/IOUT

This means approximately
2k for 20mA
4k for 10mA
8k for 5mA

A T1-1T transformer (1:1 turns ratio) can handle up to 250mW input power and
30mA DC current. The maximum output current of the AD9851 is 20mA, and with 50
Ohm termination this gives 20mW of input power. So the transformer does not
impose a limit on the output current of the DAC.

Referring to figure 9 in the AD9851 datasheet, the only case where trouble
could arise would be if the filter had a very high input impedance. The DAC has
a compliance of +1.5V maximum. If the impedance of the filter were higher than
60 Ohms OVER THE TRANSFORMER FREQUENCY RANGE 80kHz to 200MHz then the DAC
compliance range might be exceeded with a current setting of 20mA. The benefit
of the transformer coupling is that it blocks the DC component of the output
signal, so that the filter can have very high input impedance at DC without
exceeding the compliance range of the AD9851 output.

So the answer to your question depends on the impedance of the filter (compare
with figure 9) and the frequency range of the transformer. IF the filter has
moderate input impedance over the transformer bandwidth then the optimum
setting of 10mA = 3.92kOhms should be fine. Otherwise you can either reduce the
full scale output current or add a shunt resistor before the filter to reduce
the output impedance (but you lose some signal ampitude in the process)