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works at 1Mhz but not above 10Mhz

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srinivasa

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Hi

I am facing a problem in my project. i attached circuit, and datasheets.
In my circuit when i apply a signal of 1 MHZ at J2 (BNC connector) and i can see the output signal is square wave (LVDS signal) at U23 pins 2 and 3 on oscilloscope e.When i apply a signal above the 10Mhz i am not able to see the square wave but a triangular wave and voltage levels are in millivolts.the output signal should be square wave (0 to 3v with respect to GND).Help me if any one knows the solution.

Thanks in advance
srinivasa
 

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  • clock_30mhz.pdf
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  • AD8611_8612.pdf
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  • sn55lvds31.pdf
    1.1 MB · Views: 342
The 55lvds32 is designed for a differential input. You might try connecting the complementary outputs from the AD8611 to the differential inputs of the 55lvds32 to see if that helps.
 
In my circuit when i apply a signal of 1 MHZ at J2 (BNC connector) and i can see the output signal is square wave (LVDS signal) at U23 pins 2 and 3 on oscilloscope.
What is the output voltage at 1Mhz?

When i apply a signal above the 10Mhz i am not able to see the square wave but a triangular wave and voltage levels are in millivolts.
What oscilloscope are you using?
Does it have sufficient bandwidth (at least 50Mhz) to display a 10Mhz square wave?

the output signal should be square wave (0 to 3v with respect to GND).
Are you sure about that?
Have a look at the datasheet, page 6 where it states:
Vod Differential Output Voltage, Min 247mV, Typ 340mV, Max 454mV
340mV is the figure to expect.
On page 1 of the datasheet, in the FEATURES section it states:
Typical output voltage of 350mV and 100Ω load.

JimB
 
First, your schematic is incomplete. What is the value of R35? Did you read the Applications section starting on page 10 of the datasheet carefully indicating the input capacitance of the AD8611 and the impact of the source resistance in combination? An increase of the RC time of the input will dramatically increase propagation delay.

Second, is the circuit constructed on a well-designed PCB that minimizes the effect of any stray/parasitic capacitance at the input?

Third, the 5V Vcc supply for the comparator is too high to be feeding an ~5V drive to the line driver operating at 3.3V. Did you label this supply improperly?

Fourth, what are the values of R33 & R34 and what is the level and type of the input signal?
 
R33 =18.7K and R34=1k ,votage divider gives a 0.25v(at pin 3) and voltage applied at J2 is 6db (1.262Vpp,sine wave)
I am measuring the LVDS signals(each) with respect to GND. for 1Mhz output is 0 to 3.3V square wave.
The Osilloscope has 100MHz band width.
Any techniques are there to measure the signal or any circuit or procedure to follow how to measure the high frequency signals.

My requirement is to apply a 6db, 30MHZ sinewave and to get 3.3V(with respect to GND,not the differential) at the output.
 
Last edited:
Hi

I am facing a problem in my project. i attached circuit, and datasheets.
In my circuit when i apply a signal of 1 MHZ at J2 (BNC connector) and i can see the output signal is square wave (LVDS signal) at U23 pins 2 and 3 on oscilloscope e.When i apply a signal above the 10Mhz i am not able to see the square wave but a triangular wave and voltage levels are in millivolts.the output signal should be square wave (0 to 3v with respect to GND).Help me if any one knows the solution.

Thanks in advance
srinivasa
perhaps with stray capacitance and R35 in series to input
it behave like a filter at Higher frequencies.
instead
You may have impedance termination and capacitive coupling using 100pF approx at 10MHz
 
Last edited:
R33 =18.7K and R34=1k ,votage divider gives a 0.25v(at pin 3) and voltage applied at J2 is 6db (1.262Vpp,sine wave)
I am measuring the LVDS signals(each) with respect to GND. for 1Mhz output is 0 to 3.3V square wave.
The Osilloscope has 100MHz band width.
Any techniques are there to measure the signal or any circuit or procedure to follow how to measure the high frequency signals.

My requirement is to apply a 6db, 30MHZ sinewave and to get 3.3V(with respect to GND,not the differential) at the output.

That additional information above is noted. However, you did not include the value of the source resistor (Rs or R35), which I suspect was far too high given your initial problem of millivolt spikes above 10Mhz. Also left out was the type of construction of your project and the value of Vcc to the comparator. Using the information you did supply I was able to derive the value of Vcc at 5V. Further, I suspect the project is NOT on a purpose designed PCB. Please correct me if my conclusions are in error.

Taking the information provided above, I used the spice model in the data sheet and LTSpice to simulate your comparator circuit, with only a minor adjustment to the subckt pin/spice order to comply with the proper pinout of the device in the SOIC package.

I found, rather quickly, that the circuit configuration had serious limitations. First, if Rs is below the recommended minimum of 300Ω one encounters common mode problems at certain input levels. Second, the input level and/or type can cause wide shifts in the duty cycle at the output; some is to be expected for type of signal owing to delta in rise times IF the circuit is configured correctly. Third, if Rs is too high, the input RC induced time delay causes further limitations of duty cycle to the point of functional failure of output. Fourth, the AD spice model using a Vcc of 5V indicated a high output ranged from 4.64V unloaded down to an excessive load of 500Ω at 3.44V demonstrating that the high output with a Vcc of 5V would likely exceed the input limits of the SN65XXXX line driver operating at 3.3V. Fifth, a budding engineer should never take for granted bits and pieces of information contained in a datasheet...one must learn to read between the lines. By way of example, did you read any data or see any graphics on the AD sheet pertaining to 3.3V operation OR WHY the source resistance HAD to be at least 300Ω?

Pay close attention to the plot I included on the right. There are clues there to lead you to the proper design configuration if you just give it some serious thought without trying to take shortcuts. You will likely be forced to use the 5V option on the comparator so here is one last clue...think open collector buffer.
 

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  • AD8611 Datasheet Version Vcc=5V.jpg
    AD8611 Datasheet Version Vcc=5V.jpg
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  • AD8611 Datasheet Version Vcc=3,3V.jpg
    AD8611 Datasheet Version Vcc=3,3V.jpg
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  • AD8611 with Hysteresis.jpg
    AD8611 with Hysteresis.jpg
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