AD8349: Input impedance of the baseband inputs

Document created by analog-archivist Employee on Feb 23, 2016
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I'm asking about the AD8349 Modulator:
1.Can you tell me what the impedance input of the baseband Input is? And what
it means that the Baseband must be driven from a low impedance source?
2. How filter between the DAC 9767 and AD8349 influence on the impedance
matching? And why you recommended connecting a resistor between I and Q
differential inputs?
3 . How this resistor influence on the input impedance?


1. The input impedance of the baseband inputs is so high as to be negligible if
they are driven from a low impedance source like the differential amplifiers
shown in figure 44 or even the resistor networks shown in figure 51.

2. There is no impedance matching as such being done between the DAC outputs
and the AD8349 inputs. It is assumed that the distance from the DAC to the
AD8349 is short and that the lines are not required to have characterized
impedance and it is also assumed that the input impedance of the AD8349
baseband input is much higher than the resistors R1, R2 and R3.

Looking at figure 51, R1 and R2 convert the differential current output of the
DAC into two voltages. R1 = R2 and the size of R1/R2 determines the common mode
level that the signal will be sitting at. If a filter is present between R1/R2
and R3 then the filter design must take account of the non-zero source
impedance represented by R1/R2 and the finite load impedance represented by R3.
None of this has anything to do with the impedance of the Baseband input, which
is assumed to be relatively high, and therefore negligible.

3. The resistor, R3, between the true and complementary baseband inputs
interacts with the R1 and R2 to attenuate the amplitude of the differential
signal appearing at the AD8349 input terminal. When the DAC output is zero
(midscale of a sinewave) then the DAC outputs are both outputting the same
current, typically 10mA. Therefore there is 400mV across R1 and 400mV across
R2. Therefore the differential voltage is zero so no current flows through R3.

As the DAC output moves away from midscale, one voltage will increase and the
other will decrease symmetrically. The differential load resistance R3 will
shunt away some of the current being dropped through R1 and R2, leading to a
reduction in the differential amplitude of the signal.