AD8309: Overshoot atfer rise time

Question I use a AD8309 in RSSI mode. There is an overshoot after the rise time: Why ? Answer The overshoot you have observed is basically the nature of the AD8309 output stage, it takes a few moments to settle. There are one of two things to deal with this overshoot. 1) Delay the point at which you measure the output pulse. In this case you could just wait a few 100ns until the RSSI settles. 2) Place a small capacitance on the VLOG output to reduce the overshoot. Something of the order of 30-50pF should be sufficient and shouldn't have much effect on the overall response time of the AD8309.

AD8309: Limiter Amplifier

Question Could you give me more information on the Limiter Amplifier? Answer The AD8309 can provide a large voltage gain combined with progressivecompression, through which an IF signal of high dynamic range is converted into a square-wave (that is, hard limited)output, from which frequency and phase information modulated on this input can be recovered by subsequent signal processing. For this purpose, the noise level referred to the input must very low, since it determines the detection threshold for the receiver. Refer to the datasheet on page 2 for more details on the limiter amplifier.

AD8309 Limiter impedance

Q. What is the impedance of the limiter in the AD8309? ------------------------------------------------------------------------------------------- A. The limiter used in the AD8309 is an open collector circuit. The impedance is determined by the value of the pull up resistors used. The minimum resistor value is 50 ohms. Since the limiter circuit is differential, the minimum impedance would be 100 ohms differential.

AD8309 Limiter output swing

Q. As per the AD8309 data sheet the limited output VLIM = VS – 400 mV × RLOAD/RLIM (data sheet page no:17). In our design we have RLOAD=499 ohms & RLIM = 402 ohms. For this we are getting 250 mV output in each pin. It didn't match with the given relation? ---------------------------------------------------------------------------------------------------------------------------------------------- A. You are bandwidth limited due to the choices of resistors you are using and the capacitance of the test equipment. With the resistor choices and the capacitance of the measurement equipment there is a low pass corner frequency of 20 MHz. This is what is causing the voltage to drop off. If you were make the resistors 5 times smaller you achieve the same voltage swing and increase the bandwidth. This will also cause an increase in the current. The current would increase from 1 mA to 5 mA.