TRF3705: Base band input DC properties

Hi,

Donald Kay said:

We intend using the TRF3705 in an application where the baseband signal is only about 50kHz and coming from a micro via some circuitry.  All the examples show 50R resistor to ground on these inputs.  In our application burning 4 x 10mA peak of drive current in these is undesirable.

The Data sheet just gives an input impedance of 8k||4.6p.

Is there any DC current? If so how much?

The input is expected to be high impedance (consisted of bipolar differential network). With 0.25V of VCM supplied externally, your VCM must be able to source at least 0.25V/8kOhm = 31uA of current.

Donald Kay said:

The 'recommended' input signals range from 0 to 0.5V (0.25V Common mode).  Data says VCM can be up to 0.5V.  If we lift VCM from 0.25 to 0.5 do we suffer any penalty (like increased supply current)?

There is a slight decrease in P1dB with increased VCM. See Figure 7/Figure 11 for differences

Donald Kay said:

Q3

Sometimes a 'rough' schematic - at least for the bits around pin connection can be very useful.  Are these available?

Please see design resources on the TRF3705 EVM site. There are EVM schematic/layout available. Thank you.

https://www.ti.com/tool/TRF3705EVM

Thanks for the tip on the Vcm effecting P1dB.  That makes sense as there will be additional static current in the outputs of the two multipliers which will reduce the output power.

But I do not understand the explanation for the inputs.  The input bias current is normally independent of the input impedance.  For a bipolar differential pair the input impedance 2 x Beta / Trans conductance.  The bias current is emitter current source current divided by 2 divided by Beta.  Beta is current gain.  So I do not think that you can assume the bias current is Vcm/Z.  Making this assumption also suggests that the current would be flowing into the IC but if it is bipolar input it would have PNP input and the current would be flowing out.

So it would be good to get a head up from the design team as to the bias current level and bias current direction.

On the request for schematics - I was requesting schematic of the IC not an application using it. I know we seldom get (or want) full schematics but just for the pin circuits it can be very useful.

Hi Donald,

We do not have access to design team anymore to have more accurate input modeling. We can only go as far as the datasheet specification. Even with more precise modeling of bipolar input, the question would be how much more accurate you will need. For most of our applications for interface with TRF3705 and DACs, the model in the datasheet is sufficient. If you need more accurate model, I would recommend that you order an EVM and perform measurements with your TRF3705 input circuit. 

All internal schematics are TI confidential. We cannot distribute it outside of TI.