ADS54J69: ADS54J69 ANALOG INPUT

User,

Where are you measuring the 0.632Vp-p at, the ADC input? What type of probe are you using?

When you remove R709 and R717 where are you measuring this signal? When R710 and R715 are removed, are you measuring at the ADC input? 

Each leg should have 0.632Vp-p after the transformer.

Regards,

Jim

this point that I measured is not ADC input， but it is at the capacitor C705. All of measurement‘s points are at the capacitor C705 in different circumstances。

the type of the probe oscilloscope is passive voltage probe. And the channel A and channel B is the same situation。

the exact location is between the capacitor C705 and J701。

User,

On the TI EVM, with a 10MHz input tone at 1.23V p-p, I get 0.641V p-p at both ADC inputs. This board is using the same circuit as you show in the post. This is the expected voltage as these differential voltages will get added inside the part.

Regards,

Jim 

I want to know where the point that you measured is.

you mean this phenomenon is normal? the input signal can be pulled down to 0.632Vp-p at the input end J701?

Maybe I explained unclearly.

I introduce my steps detailedly：

1. First my input signal is sine wave signal，10Mhz,1.23Vp-p；

2. I insert RF cable into the connector J701；

3. On this condition, i measure the input signal at J701, it is 0.632Vp-p。this is not correct.

4.I disassembly the resistors R709 and R717, the input signal is also changed。it is a half.

5.But,when we disassembly the resistors R710 and R715, the signal is normal  and Vp-p can arrive the 1.15Vp-p. 

6.From above these steps, I found the resistor R710 adn R715  influence the input signal.

I know the signal can be changed into a half of input signal through thetransformer。That is to say，at the pin 4,6 of the transformer T702, the signal should be a half of input signal。but the input signal should not be changed。

my question is why the input singal can be changed in due to R710 and R715.

Panxiaohong

Hi Jim：

The board is not powerd up。

This should be affected by resistors R710 and R715. When the board is not power up，disassembly the two resistors，the signal Vp-p can arrive the 1.15Vp-p that is almost equal to input signal.

I try to change the value of the two resistors from 24.9ohm to 51ohm and 100ohm, the signal Vp-p can become great, the Vp-p=0.842V when 51ohm, the Vp-p=0.990V when 100ohm.

But I dont know whethers this adjustment  is appropriate or not.

thanks

panxiaohong

Hi Jim:

When we do the measurement, need we resistor R710 and R715? 

Yes.

Yes

Hi Jim:

I do the measurement when the board with resistor R710 and R715 is powered up, with a 10MHz input tone at 1.23V p-p, I get 0.303Vp-p at both ADC inputs.

I think this is not relation to whether the board is powered up or not。

Because input signal is already pulled down to Vp-p 0.632V due to R710 and R715.

This is why? why the input signal can be pulled down?

Thanks

panxiaohong