TINA/Spice/THS4521: the output scope is very bad

user4973896

Part Number: THS4521
Other Parts Discussed in Thread: TINA-TI, , ADC3441, TIDA-00499

Tool/software: TINA-TI or Spice Models

Can anyone tell me what causes the waveform to be so bad, what can be improved to make the waveform normal?ths4521_TIna_NO cap gnd RIN 10V.TSC

over 7 years ago

0 kai klaas69 over 7 years ago

Guru 140200 points

Hi,

your circuit cannot work because you destroy the symmetry in the feedback paths.

What are you planning to do?

Kai

0 user4973896 over 7 years ago in reply to kai klaas69

Expert 1555 points

I now want to record a 40Mhz 450V transient voltage，you say i destory the symmetry in the feedback paths,how should i adjust the ciruit?

0 kai klaas69 over 7 years ago in reply to user4973896

Guru 140200 points

Kai

0 user4973896 over 7 years ago in reply to kai klaas69

Expert 1555 points

i want to interface to adc3441,so the vref is 0.95V ,not 1.65V,is there a document explain the value of the resistors in the circuit like this？

0 Sean Cashin over 7 years ago in reply to user4973896

TI__Mastermind 23226 points

Figure 54 in the datasheet explains how to keep the amplifier balanced for a single ended input, by using a resistor equal to the source impedance. You need to keep the impedance equal between each input and ground.

With a 0.95Vocm, you will have a maximum output swing to the rail of 1.5V. At unity gain, that means you need at least 1.5V/450V attenuation at the input. You could make a resistor divider similar to what Kai suggested, and or you could use a high frequency step down transformer, which would also provide galvanic isolation between your high voltage and low voltage circuits.

ths4521.TSC

0 user4973896 over 7 years ago in reply to Sean Cashin

Expert 1555 points

as you see ，the rg1 is become smaller if i want to increase the input range to 3500V 。I find a reference design like pic above，which is from TI reference design TIDA-00499,but as you can see it use a zener act as the RT role ，and at such high voltage input ，the RG can be still hundreds Ω ，now i am confused，can you tell me how does this zener work？i guess，this zener may be use to be a resistor except it's protect function。besides ，if i want to record 40Mhz bipolar signal ，can i still use this zener？

when i use zener to simulate，i find the zener has a lot of parameters。i guess whether the RZ parameter of the zener acts the RT role ？ can you answer me？

0 kai klaas69 over 7 years ago in reply to user4973896

Guru 140200 points

Hi,

this is no zener diode but an ESD protection diode with very low capacitance. And the ESD protection diode isn't part of the voltage divider! The voltage divider is performed by R3 + R4 + R5 + R6 + R9 and the parallel circuit of R1, R2 and the input impedance of THS4521 circuit.

Kai

0 user4973896 over 7 years ago in reply to kai klaas69

Expert 1555 points

the circuit above is not standard circuit ,how calc the other value of R8 R22 and else?

0 Micah B Brouwer over 7 years ago in reply to user4973896

TI__Intellectual 1910 points

Hello,

As I said in a previous post, Rs and RT are not located in this circuit. Are you asking how to choose the value of R8 and R22, and why they are different? This will be answered in this thread.

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TINA/Spice/THS4521: the output scope is very bad