THS4151ID Overheating

Hi Mohit,

I know you gave a detailed description of your circuit, but a schematic would be helpful.  When I look at the datasheet for the AD5293 I see very large values of capacitance on the analog inputs (~85 pF).  These values of capacitance are high enough that they will impact the circuit performance at higher frequencies. 

Also, the input signal is very large.  Do you know how much power the input resistors need dissipate?  Will this heat travel to the amplifier?  What temperature is the amplifier rising to?  What temperature were you expecting? 

What is the amplifier supply voltage? 

Are you using the DGN package? This package has the best thermal dissipation properties. 

Many times when an amplifier overheats it is either too much load current or the amplifier is oscillating.  I can't tell in this case. Can you post a scope plot of the output signal? 

The filter says it's a 50 Ohm impedance, why are you using 25 Ohm resistors?  Have you tried 50 Ohm resistors between the amp and the filter? 

Regards,
Loren

Hi Mohit,

I should have been more clear. I would strongly recommend against using a digital pot as a feedback or gain setting resistor. Because of the parasitic capacitance on the nodes the resistance at high frequencies will be primarily capacitive and will be far less than the desired value.

If you look at the AD5293 datasheet Figures 25,26 and 28 you will see this part was not characterized for operation above 1MHz, and even at 1MHz the parasitic capacitance is showing an influence on performance.

You will need to find another way to set the THS4151 feedback resistance.


Regards,
Loren

Dear Loren,

THANKS FOR REPLYING,

• I HAVE TRIED WITH 50 OHMS RESISTORS, IT IS NOT MAKING ANY DIFFERENCE.

• SCHEMATICS ARE ATTACHED WITH THIS MAIL. ONE THING I MISSED TO TELL YOU, THE OUTPUT OF FILTER IS GOING TO AN ADC (ADS850) WHICH HAS 47pf CAPACITORS AT INPUT NODES.
• I HAVE ALSO ATTACHED A FILE WITH THIS MAIL CONTAINING RESPONSES CAPTURED ON CRO WITH DIFFERENT RESISTOR TYPES.
• I CAN NOT MEASURE THE TEMPERATURE OF THS4151, BUT IT IS AT LEAST 60-70C, MY PRIMARY CONCERN IS NOT HEATING UP OF IC, I SUSPECT THIS HEATING UP OF IC IS CAUSING RINGING IN THE OUTPUT SIGNAL WHICH YOU CAN SEE IN THE CRO TRACES ATTACHED.
• I HAVE ALSO TRIED WITHOUT PUTTING ANY DIGITAL POT AND NOT EVEN Rt RESISTOR, THIS DOES NOT MAKE ANY SIGNIFICANT DIFFERENCE IN HEATING OR NOISE PERFORMANCE.
• ONE THING IS VERY UNUSUAL, NOISE AT OUTPUT OF FILTER IS MORE THAN TWICE AS AT INPUT TO IT. 
• RESISTORS AT INPUT ARE NOT HEATING UP.
• YOU HAVE MENTIONED, SIGNAL AT INPUT TO THS4151 IS LARGE. HOW CAN THAT BE SIGNIFICANT IF IT IS WITHIN THE DYNAMIC RANGE.    
• SUPPLY TO THS4151 IS +/- 15V.
• PARNUMBER FOR THS4151 USED : THS4151ID. YOU CAN FIND MORE ABOUT IT AT THIS WEBLINK: 

• http://www.digikey.com/product-detail/en/THS4151ID/296-10054-5-ND/380601

  RESPONSE 2 TO E2E.pdfFIGURE_1_THS4151.pdfFIGURE_2 _ ADS850.pdf

Hi Mohit,

High temperatures should not cause ringing in a properly designed circuit.   With 30V supplies and 17.5mA of standing current the amplifier dissipates over half a watt with no input or load current.  I would expect the amplifier to run pretty warm, so 60 to 70 degrees C sounds fine. 

That output waveform captures dont look like nose.  It looks like an oscillation at ~28MHz.  Can you try putting larger resistors in for R90 and R490 (50 Ohms to start).  The 47pF capacitors are pretty large to have on the output of the amplifier and the ADC has 20pF of internal capacitance.  High speed amplifiers have difficulty with capacitive loads.  The solution for this is to use larger isolation resistors between the amp and the capacitive load. 

As I stated earlier the filters are specified for a 50 Ohm environment.  They are driven with a 25 Ohm source and there appears to be no termination at the output.  Filters can cause ringing when they are not terminated properly.  Filters also can give voltage gain, especially when the input impedance and output impedance are not balanced.  In this case it looks like the filter is providing significant voltage (and noise) gain.  The filter is also doing an impedance transform betweent he amp and the ADC. 

One good experiment would be to  replace C77 and C62 with 50 Ohm resistors.  Along with putting 50 Ohm resistors at R90 and R490 this should go a long way towards optimizing the filter response and the amplifier output loading. 

Best Regards,

Loren

Hi Mohit,

Can you attache a photo of your PCB? 

Potentiometers are usually not suitable for use in high speed circuits. 

I can give general advice, but it is very hard to degug a circuit remotely. 

Regards,

Loren