Calculating RF Choke Inductance on AD8353

For the RF gain block, the AD8353, on page 13 of the datasheet (Rev  C), there is discussion of the need for a RF output choke between the power supply and the RF output when operating at supply levels below 5V. It suggests using a 100 nH inductance for high frequency operation.and care should be taken to ensure that the lowest series self-resonant frequency of this choke is well above the maximum frequency of operation for the AD8353.

Then it says "For lower frequency operation, use a higher value inductor."

How is this calculated for different supplies and frequencies.

In my case 3.3 V supply and 400-450 MHz RF.

Can you give the general procedure and an example solving for my case please?

Thanks

Parents
  • 0
    •  Analog Employees 
    on Feb 19, 2010 6:18 PM over 11 years ago

    When using a pull up choke as described in the AD8353 datasheet, You want the impedance presented by the choke to be sufficiently high enough so it doesn't interfere with the circuit.  at 400 MHz - 450 MHz the reactance presented by a 100 nH inductor is 251 ohms - 283 ohms. (XL= 2pi* F*L) This impedance is much greater than the characteristic 50 ohm impedance of the AD8353 and will not impact the circuit performance. Another parameter of concern is the the first series resonant frequency of the inductor. This the first at which the inductor starts to look like a capacitor instead of an inductor. ( you would start coupling energy out of you circuit) This should be higher than your planned operating frequency. In the case of the AD8353, take a coilcraft 0603CS 100nH inuctor, looking at the specifications we find the min SRF is 1.4 GHz This ~ 3 times greater than our highest planned operating frequency and should have no effect on circuit operation.

Reply
  • 0
    •  Analog Employees 
    on Feb 19, 2010 6:18 PM over 11 years ago

    When using a pull up choke as described in the AD8353 datasheet, You want the impedance presented by the choke to be sufficiently high enough so it doesn't interfere with the circuit.  at 400 MHz - 450 MHz the reactance presented by a 100 nH inductor is 251 ohms - 283 ohms. (XL= 2pi* F*L) This impedance is much greater than the characteristic 50 ohm impedance of the AD8353 and will not impact the circuit performance. Another parameter of concern is the the first series resonant frequency of the inductor. This the first at which the inductor starts to look like a capacitor instead of an inductor. ( you would start coupling energy out of you circuit) This should be higher than your planned operating frequency. In the case of the AD8353, take a coilcraft 0603CS 100nH inuctor, looking at the specifications we find the min SRF is 1.4 GHz This ~ 3 times greater than our highest planned operating frequency and should have no effect on circuit operation.

Children
No Data