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TLV9062: Are C5/8 of OPAMP evaluation board mandatory parts?

Kazuya Nakai59
Guru 20140 points
Part Number: TLV9062
Other Parts Discussed in Thread: DUAL-DIYAMP-EVM, , TINA-TI

Hello guys,

One of my customers is evaluating  TLV9062 using Inverter Amplifier type (Figure 11. Inverting Amplifier Schematic) of TI OPAMP evaluation board, DUAL-DIYAMP-EVM.

https://www.ti.com/jp/lit/ug/sbou193/sbou193.pdf

Could you please give me your reply for them?

They use the following parameters for the evaluation.
R1/6=Open, C4/7=33nF, R3/8=3.9kohm, C3/6=22pF, R2/7= 390kohm, R4/9=560kohm, R5/10=560kohm, C5/8=100nF, C1/2=100nF

Also each terminal voltages are below.
V+=2.2V, V-=GND=0V.

And REFA/B=2.2V supplied from a power supply separated from V+ power supply,

They took a gain-frequency characteristics in these conditions. As the result, the characteristics was almost same as TINA-TI simulation result when 100nF are mounted on C5/8.
But when the 100nF were removed from C5/8, the gain in range over 5kHz became lower than TINA TI simulation result.

There question is the follow.

Q1. What do you think the cause?

Q2. Are C5/8 mandatory parts?  

Your reply would be much appreciated.

Best regards,

Kazuya.

  • 0
    Guru 140200 points

    Hi Kazuya,

    yes, there's a difference but it is only very small:

    kazuya_tlv9062.TSC

    The reason for the difference is that the -input of OPAmp is seeing different stray capacitances to signal ground when C2 changes. Similar to this:

    This has an impact on the frequency response and the common mode rejection.

    It's not wise to remove C2 all the way. C2 in combination with R3 and R4 makes a good noise filter. Keep in mind that any noise on the +input of OPAmp (coming from "V2", for instance) will be amplified by the noise gain of this circuit which is up to (390k/3k9 + 1)V/V = 101V/V. Because of this, decreasing the values of R3 and R4 can additionally make sense. Keep in mind that they also produce noise, called "thermal noise" or "Johnson noise".

    Kai

  • 0 in reply to kai klaas69
    TI__Genius 9350 points

    Hi Kazuya, 

    Yes, Kai is correct in that C5 is good to be used as a noise filter. 

    On Page 9 of the user's guide (https://www.ti.com/jp/lit/ug/sbou193/sbou193.pdf) also mentions this as shown below. 

    Also, please note that simulation models such as TINA-TI are based off of "typical" specs and are provided for customer's convenience but do not replace real world testing that should be done. 

    Best Regards,
    Ashley

  • 0 in reply to Ashley Kang
    Guru 20140 points

    Hi Kai, Ashley,

    Thank you very much for your reply and I'm sorry to be late response.

    The customer monitored voltage waveform at IN+ terminal of TLV9062 without C2 capacitor.
    Then a sine wave with an amplitude of about 1 mVp-p which had the same frequency as the output of the amplifier was observed. 
    (Since the input is a sine wave of 5 kHz, the amplified output is also a sine wave of 5 kHz).  

    Do you know why the sine wave at IN+ terminal was generated?
    V2(2.2V power supply) voltage connected to R3 is stable completely because it is supplied from another power supply from V1 and bulk capacitance is connected near by R3.

    Also when C2 capacitor was connected to IN+ terminal, the sine wave was disappeared.

    Is the cause in TLV9062? Is the sine wave generated by TLV9062?

    Could you please give me your reply?

    Thank you again and best regards,

    Kazuya.

        

  • 0 in reply to Kazuya Nakai59
    Guru 140200 points

    Hi Kazuya,

    keep in mind that the TLV9062 has a finite gain. So it will need a small differential input signal to make the output show a big signal. This differential input signal is very small but it cannot be zero.

    Also, there's an impedance between the inputs, mainly the differential input capacitance. And the small differential input signal will cause a small signal current flowing through this differential input capacitance. And this current will cause a voltage drop across the 560k resistors. Adding C2 will make this voltage drop decrease to zero.

    Kai

  • 0 in reply to kai klaas69
    Guru 20140 points

    Hi Kai,

    Thank you very much for your reply.

    I understood it.

    Thank you again and best regards,

    Kazuya.