THS4509: Full scale differential Output

Hi Harsh,

This device outputs around 1.1V from the supplies, which would mean the max voltage at each output is around 1.4 volts. With this common mode and Vopp the device would have to adjust the outputs resulting in a shifted common mode voltage to accommodate for the lack of headroom. One quick fix if possible is to skew the supplies on the device to give more headroom for the device to fully swing to the 1.45V max required. If skewing the supplies is possible, something like -1.5V to +3.5V would be good as this will ensure the device is operating around mid-supply.

Best Regards,

Ignacio

and scale your R values up, that is heavy load to the FDA outputs, maybe more in the 300ohm Rf region, also add at least series output RC to the ADc

According to this circuit i'm not getting 2Vpp as per 2.22 gain setting 

for Recheck

Input AC coupled supply Freq 3.5Mhz and Amplitude is 450mV

can you please provide appropriate value for my application.

Hi Harsh,

At the higher frequencies, the capacitors to ground will behave like a short and the 65ohm and 50ohms in parallel will add to Rg resulting in a lower signal gain. I was able to adjust the circuit and have attached it below to get 2Vopp. 

Best Regards,

Ignacio

3858.THS4509_ADS5294.TSC

Thanks ignacio for the support,

but my input is a 0.9Vpp so i think my amplitude should be 450mV in input source.

Can you please provide with this input.

Best regards,

Harsh

Hello Ignacio,

Thanks for the support and solving a issue.

Best Regard,

Harsh 

Hello Ignacio,

I have one more question on this, can i replace THS4509 with THS4551 part if YES,

can you please provide simulation file as per my previous requirement,

Best Regard,

Harsh

Hi Harsh,

I would recommend staying with the THS4509 as the THS4551 is likely to slow for this fast of an ADC. The ADC EVM implements the THS4509 in its design because it has the speed to drive this device appropriately.

Best Regards,

Ignacio

Hello Ignacio,

I'm staying with the THS4509 for my design, but I just want to understand the working and function of the THS4551 with ADS5294 for my knowledge. So, could you please provide a simulation for my previous requirement with the THS4551?

Best Regards,

Harsh

Hi Ignacio,

We add a ADS5294 Input model in the THS4509 simulation that you provide us after the simulation 

Above is the output signal with ADS5294 Input model,

But why in this i'm seeing a shifting in differential output in Vout_ADC point ?

Above is the simulation circuit with ADS5294 input model

I have one more question If i set the power supply +3.3V and -1.8V for THS4509 so is its okay for this part because in datasheet recommended operating is giving MAX 5.25V. So is its okay to use +3.3V and -1.8V for THS4509 as same circuit, 

Best Regard,

Harsh

Hi Ignacio,

Thanks for providing an example circuit of the THS4551.

I will wait for your simulation circuit with the addition of the capacitor at the output and charging up.

Best regards,

Harsh

Hi Harsh,

Sorry for the confusion, just to clarify when I tried adding the additional stage the simulation would break. I assume the simulation could not find an appropriate bias point and could not converge. Is the output value you are seeing, correct? In terms of the amplitude and frequency of the signal. Ignoring the slight shift.

Best Regards,

Ignacio

Hi Ignacio,

The output value is correct that is 2Vpp, and the input supply  Amplitude should be 450mV and frequency range should be 3.5MHz o 10MHz.

Best Regards,

Harsh

Hi Ignacio,

So I can ahead with THS4509 fully working simulated circuit for my design right ?

And can you provide me your recent simulated circuit ?

Best Regards,

Harsh

Hi Harsh,

The simulated file I previously sent is the latest version I have. I would also like to highlight the ADS5294EVM as a reference. The EVM includes the THS4509. 

Best Regards,

Ignacio

Hi Ignacio,

Thanks for your support,

Best Regards,

Harsh.

Hi Harsh,

The slight change at the output seems to be from the added components which will not perfectly model the ADC when it is sampling. We always recommend system level testing as simulations will only show so much and can often show behaviors that will not appear in the real system.

Best Regards,

Ignacio

Hi Ignacio,

I got it what you are saying

Best Regards,

Harsh