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THS4032: Parallel OP-AMP with single source

Fairuz
Prodigy 200 points
Part Number: THS4032
Other Parts Discussed in Thread: LMH6559

Hi, I am currently using THS4032 to design my ADC input circuit. the problem is i only have limited source (ATE tester resource) but multiple test site required. what is the maximum number of site i can run in parallel? Spice simulation doesn't seems issue but afraid it will affect on live circuit. attach is my circuit. what other parameter i need to check?

  • 0
    Guru 140200 points
    Hi Mohd,

    this mainly depends on the source impedance of V7. As each THS4032 has an input impedance of 1M, an input capacitance of 1.2pF and an input bias current of 3µA this can result in a voltage division in combination with the source impedance of V7 and can also create an additional offset voltage.

    Do have any cabling at the inputs of THS4032? In this case you should use 50R or 75R technique.

    Kai
  • 0 in reply to kai klaas69
    Prodigy 200 points
    Hi Kai, the V7 comes from ATE instrument and will go to THS4032 through ~5inch copper trace 50mils wide (1meg shunt resistor will be near THS4032). the source characteristics at 50kHz band is 1.5ohm (DC), and 50ohm(DC) at 80MHz band. with low source impedance, will it cause issue if i connect to multiple THS4032?

    I am not familiar with 50R and 75R technique. any reference i can look at?
  • 0 in reply to Fairuz
    Guru 140200 points
    Hi Fairuz,

    yes, with a low source impedance the loading effects should be minimal.

    What is your signal bandwidth? Does the signal contain steep edges?

    Refering to the 50R technique, search in the internet for "transmission line", "characteristic impedance", series termination", "parallel termination". The datasheet of LMH6559 also contains useful information.

    Kai
  • 0 in reply to kai klaas69
    TI__Mastermind 20586 points

    Hi Fairuz,

    It may work outright, but I would avoid the risk of loading the signal and use an interim gain=2 amplifier, with impedance matching at the input, between your source and test site amplifiers, with a 100ohm isolation resistor for driving all that input capacitance. The max limit is then determined by how many 1.2pF increments vs. a 100 ohm isolation resistor will keep the resulting RC filter pole above the desired bandwidth.

    Best regards,

    Sean

  • 0 in reply to kai klaas69
    Prodigy 200 points

    Hello Kai, we are using 80MHz bw, should be 50ohm source impedance. the signal is for 12bit ADC input, which the source will ramp 3V/1ms.

    one more thing, i read if the input is leave floating (which it may disconnect from source), the opamp output may follow VDD/VSS supply. So probably change 1M input resistance to 10K will help? or there is other alternative since there will be large input offset if using 10k resistor.

  • 0 in reply to Sean Cashin
    Prodigy 200 points
    Hi Sean, i not sure i can understand you. do you mean changing the 30ohm(from sch) to 100ohm resistor? and does the 1.2pf increment refers to 2.2nF load cap? it is not really amplifier, merely like a buffer since i only use 1 source to source 10 devices at the same time. i need to what is the maximum opamp i can place and if there are drawback.
  • 0 in reply to Fairuz
    Guru 140200 points

    Hi Fairuz,

    if the input can be left floating, I would use 50R technique and introduce an additional buffer. I think it's the same what Sean is recommending:

    On the left the ATE can be seen with its 50R output impedance. U1 is an input buffer with an input impedance of 50R. The connection between the ATE and U1 should be made with a 50R cable or a 50R transmission line in microstrip technique. U1 has a gain of 2 to recover from the 6dB damping resulting from the voltage divider R3 and R4. The output of U1 is not directly connected to the other OPAmps from which only four are shown for clarity. This is done to isolate the output of U1 from the capacitive load of the wiring. The OPAmps on the right side do also have a 51R resistor directly at their + inputs. This is to dampen resonances which can result from inductances and capacitances of the wiring between the OPAmps. These resitors can enormeously help to stabilize the whole circuit.

    You will notice that I have reduced the feedback resistors from 1k to 300R. These are the recommended values for a gain of 2. Have a look into the datasheet of THS4032.

    I need not to mention that the distance between the many OPAmps should be a short as possible...

    Kai

  • 0 in reply to Fairuz
    TI__Mastermind 20586 points
    Hi Fairuz,

    I agree with Kai. Thank you Kai for providing a circuit. To confirm: R3 provides input impedance matching; U1 is in gain=2 to restore the attenuation caused by impedance matching; R5 is the isolation resistor I was referring to, between U1 and the parallel input capacitance.
    Kai has split the 100 ohms up, which will allow more parallel amplifiers (25 vs. 13) and still maintain 80MHz of bandwidth.
    I came up with these numbers by calculating the bandwidth of the RC (R=51, THS4032Cin=1.5pF) filter between U1 and a U2-U(N) input, and solved for N to get 80MHz. Additional amplifiers will just add another RC pole and progressively decrease the bandwidth below 80MHz.

    A bigger concern may be the 30 ohm, 2.2nF RC filter you have. That RC bandwidth is only 2.4MHz. Even if you optimized the cap drive BW and decreased the 30 ohm to 5, the bandwidth is still only 17.8MHz. In order to pass 80MHz with this amplifier, you will need to decrease the 2.2nF to at most 91pF, with at least 15 ohms of isolation.

    Best regards,

    Sean
  • 0 in reply to Sean Cashin
    Prodigy 200 points

    thanks Kai and Sean!, you guys help a lot with detail information. i forgotten to mention the 80MHz bandwidth is source capability but i am using below 2.4MHz with 30ohm/2.2nF for 12 bit ADC input.

    you are right the feedback resistor 300 ohm should be the right value according to datasheet gain = 2. but if the actual bandwidth is below 2.4MHz, i believe should not be issue with any value?

  • 0 in reply to Fairuz
    Guru 140200 points

    Hi Fairuz,

    a phase stability analysis shows that increasing the feedback resistances from 300R to 1k will erode the phase margin and can make the THS4032 oscillate...

    Kai

  • 0 in reply to kai klaas69
    Prodigy 200 points
    Hi Kai, thanks for the input. i will get 300ohm then, need to study more on oscillation.