ADS54J20EVM: The calculation of equal input impedance

suy

Part Number: ADS54J20EVM

Hi Jim,

I still have some doubts,please help me confirm this：

1， In the section 9.2.1.1，as long as i choose the frequency of input signal，i can select the corresponding RC value and calculate the ZIN

2，At this step i’m not sure whether to use the R&C value to calculating by theR&C off-chip and ZIN

For example，if the requirement of design is 100Ohm input and the frequency is 100MHz，then we can confirm the RIN is 900Ohm and CIN is4.75pF，so the ZIN equal to RIN || CIN and the value is 244Ohm。

Therefore in order to get the 100Ohm input impedance the R value is 169Ohm（100Ohm = 169Ohm || 244Ohm），is that correct？

over 5 years ago

0 jim s over 5 years ago

TI__Guru* 91947 points

Suy,

This would be true if you were driving with a DC circuit. This is not true with the image you show as it is AC coupled.

Regards,

Jim

0 suy over 5 years ago in reply to jim s

Expert 1055 points

Jim，

So what you mean is setting the RC vaule for AC coupled only according the curve of figure 133&134？

0 jim s over 5 years ago in reply to suy

TI__Guru* 91947 points

Suy,

No. DC coupled circuit. From the data sheet:

"When designing dc-driving circuits, the ADC input impedance must be considered. Figure 133 and

Figure 134 show the impedance (ZIN = RIN || CIN) across the ADC input pins".

The ADC parameters should be taken into consideration when determining the overall impedance of your input pcb traces.

Regards,

Jim

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ADS54J20EVM: The calculation of equal input impedance