Differential Impedance

[Ferdy88]

Hi,
I have a problem with the datasheet of MAX2830 (Maxim Integrated)

On page 23 in section "Transmitter I/Q Baseband inputs", it is written: "The differential analog inputs of the transmitter baseband amplifier (TXBBI+, TXBBI-, TXBBQ+, TXBBQ-) have a differentials impedance of 20 kΩ||1 pF."
Now, what do you mean by "differential impedance of 20 kΩ||1 pF" ?

I thought in a first moment that it means the input impedance of each input of transmitter (TXBBI+, TXBBI-, TXBBQ+, TXBBQ-), but after reading this article,
https://www.google.it/url?sa=t&rct=j&q=&esrc=s&source=web&cd=5&ved=0CD4QFjAE&url=http%3A%2F%2Fwww.ultracad.com%2Farticles%2Fdiff_z.pdf&ei=hhU3UvjcHdKQ7Ab424HoAg&usg=AFQjCNGVW8bGV9_foWOTChCX29ZMSi_1kg&bvm=bv.52164340,d.bGE
i now think that it is the impedance Zdiff=2*Z0*(1-k)=2*(Z11-Z12) that allows to match a balance line as described in the paper above. I want some confirmation. If it is, then I have to design two lines with a suitable characteristic impedance and k coupling.............

 

[albbg]

Simply means that between TXBBI+ and TXBBI- you will see an impedance of 20 kΩ||1 pF. The same between TXBBQ+ and TXBBQ-.

 

[Ferdy88]

Thank you albbg,
Now i have another question, I have to interface the MAX2830 (Transceiver) with the MAX5866 (dual ADC, dual DAC).
In datasheet of the , in the page 20,
it says: " Also, the DAC analog outputs are designed to drive a differential input stage with input impedance ≧ 70 kΩ."
Therefore, It means that my DAC can drive a impedance between ID+ and ID- ≧ 70 kΩ and the same applies between QD+ and QD-.

Now i want a help about how to interface the dual DAC of the MAX5866 with the input TXBBI+, TXBBI-, TXBBQ+, TXBBQ- of the transceiver MAX2830.
I know from the datasheet that the DAC is characterized by:
Full-Scale Output Voltage V_FS=±400mV
Output Common-Mode Range Min=1.29 V and Max 1.5 V
It it is capable to drive differential input stage with input impedance ≧ 70 kΩ

while the MAX2830 has a transmitter characterized by:
differential impedance between TXBBI+, TXBBI- and between TXBBQ+, TXBBQ- equal to 20 kΩ||1 pF.
the inputs require an input common-mode voltage of 0.9 V to 1.3 V.

Now It is evidently that I can't directly connect the two Integrated, now I must to find a circuit to match the output of the DAC with the input of the transmitter......
Can you give me some advice?


I use a couple of OPAMP

 

[albbg]

I can suggest you to use a fully differential amplifier in-between like, for instance, the AD8139 (or equivalent also available from other brands). If you go to the internet site od the Analog Devices you can find a very useful tool to design the differential amplifier.

 

[Ferdy88]

ok, now I'll look. I was thinking to use for each connection an OPAMP used as buffer with unity gain but I must find a way to shift the common voltage of the DAC in the acceptable range of 0.9 V to 1.3 V for transmitter. now I try the solution with the differential amplifiers.
Thank you, I'll let you know.....

 

[erikl]

... I must find a way to shift the common voltage of the DAC in the acceptable range of 0.9 V to 1.3 V for transmitter. now I try the solution with the differential amplifiers.

You'll need a reference voltage to manage this. With a ref. voltage of, e.g., 3.0V and input resistors of 100kΩ you'd get this circuit: