50 Ohm resistor at SMA connector

Question

I am designing a schematic with TI ADC12DJ3200.

This ADC is differential 100 Ohm input impedance ADC.

In Eval Board of this ADC a Balun is used to convert single ended input from SMA to differential to ADC as shown below:

Signal from SMA is directly connected to input of Balun.

But sometimes I see that a 50 Ohm pull down is placed at SMA and then a capacitor and then signal is connected to Balun as shown below as example although pull down resistor in this example is DNP: Which configuration should I use in my schematic?

Does this 50 Ohm pull down at SMA helps to see the balun a 50 Ohm impedance at input so that 100 Ohm impedance can be maintained at Output of balun?

I have seen that in some schematics a 50 Ohm pull down is used at SMA and in some schematics no pull down is used at SMA.

I am confused how to know when to use a 50 Ohm pull down at SMA and when to not?

Answer 1

Both Baluns are spec'd to have a 50 ohm input impedance, with a 1.5 VSWR (typ), corresponding to a return loss of ~-14.dB. Assuming the source (the thing driving into J2) is matched to 50 ohms, and the interface from the SMA connector to the balun is 50 ohms, I see no use for the discrete ~50 ohm resistor at the SMA.

Answer 2

Eval boards usually have provision for different circuit configurations. Sometimes they are intended for alternate uses, sometimes they are just for test and bring up purposes.

In your second example, R301 should not be installed if the signal is coming from J301. The assumptions is that the impedance is defined by the signal source output impedance, which is expected to be 50Ω.

I see two possible scenarios the board designer could have had in mind to populate R301:

• test the rest of circuit under the no-input signal condition with no external cable attached. For instance: measure noise floor, measure the impedance at the ADC input.
• characterize the input signal coming from J301 as it enters the board, by populating R301, not populating C301.

(if the external source is high impedance, R301 could help bring the impedance as seen by the balun close to 50Ω. However, the voltage attenuation would be significant, what makes this an unlikely scenario)

Answer 3

If the device connected to the SMA presents a 50ohm load then the resistor is not needed.

If the device connected to the SMA presents a high impedance then the resistor is needed to set the combined input impedance to the balun to something like 50ohms.

If the device connected to the SMA presents a DC voltage then perhaps the C301 capacitor is needed.

So it depends on what you connect to the board at the SMA.

Answer 4

√ The 1st example has an interface with a Vcm=0 due to the ADC12DJ3200 specs.

X The 2nd example has an interface with Vcm \$\ne\$ 0 for common mode voltage some other ADC. As a consequence it suffers some loss for the same impedance unlike above.

You can follow 1st example since you are using an IC that works at 0Vdc for Vcm