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Do the differential signals input need 120R?

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tony_lth

tony_lth

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The following is the design for phase shifter control signal.

DS26LS33MJ/883 is to converter diff signals to single end. Pin8 and Pin12 connect to GND.

Now I have problems:
1. Is the 120R between the diff input needed?
2. Now I measure the "phasebit7" only have 2.2V, it should be more than 4.3V. What maybe the reasons?

Thanks for comments.
 

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I had removed the 120R, the results don't change.
And I had replaced the DS26LS33MJ/883 chip, and the results don't change.
Is it because that I measued with "phasebit7" open, i.e. no connection?
 

terminating resistor is always mandatory and good practice to be used.... Usually it is 60 ohms or 120 ohms used at both ends....

remove the resistor and check the voltage level once.. if you get 4V or still get less voltage?????????
 
Hi, ckshivaram,

thanks for your reply. I removed the resistors and still got the 2.2V voltage at the single end. Could you pls give some comments?
 

Hi tony,

are you measuring the voltage by connecting the device on the other end of this IC in picture or the other end is open???

If you have any IC connected to the other end of these lines .. please remove it and measure....
 
Yes, I had removed the IC of the other end, so Pin3/Pin13/Pin5 are open when I measured, and the voltages of them are all 2.2V for the high level, and o.2V for low level. So the high level seems not normal.
 

Please remove the DS26LS33MJ chipfrom the socket and measure at those pins with respect to ground... in the absence of DS26LS33MJ it should give 4 or 5V...

---------- Post added at 18:11 ---------- Previous post was at 18:09 ----------

try reducing it to 60ohms and try this also....
 
Hi, ckshivaram,

In the absence of DS26LS33MJ and without the resistors, Pin1 and Pin2 are only 2.5V with DVM. Is the voltage right?

---------- Post added at 13:46 ---------- Previous post was at 13:45 ----------

I can't try the 60 ohms because now I am in home, I can try it tomorrow.

---------- Post added at 13:48 ---------- Previous post was at 13:46 ----------

I can't try the 60 ohms because now I am in home, I can try it tomorrow.
 

I don't think you use both pin 4 and 12, you choose one and enable the chip.

Are you making sure one of the differential input pins is higher than the other pin? If they are open circuited, all bets are off.

Your output current is less than 50 ma?
 
Last edited:
The output voltage can make a ringing because of imternal capacitors and that's why a damping resistor can be used.. have you checked with scope ??
 
Thank all of you.
I don't think you use both pin 4 and 12, you choose one and enable the chip.
Click to expand...
I connect pin4 to +5, and Pin12 to GND. Should I only connect Pin4 to +5V, and make Pin12 open? Then the chip maybe disable.
Are you making sure one of the differential input pins is higher than the other pin? If they are open circuited, all bets are off.
Click to expand...
Yes, they are open circuited when I measured.
Your output current is less than 50 ma?
Click to expand...
Yes, the output are just control bit for phase shifter, and for each bit, the phase shifter consumps about 30mA from the power supply of the phase shifter, not from the DS26LS33. And power consumption of each DS26LS33 chip is about 50mA@+5V.
please give the enable signal and see if the voltage changes or it stays permanently at 2.5V.........
Click to expand...
OK, I will try later.
.. have you checked with scope ??
Click to expand...
I will try later.
 

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