AD8347: How to use the AD8347 for demodualtion of a 100Hz to 5MHz signal

Question 1. I need to know what is the Base Band frequency range of the AD8347 ? [AE] 60MHz. 2. My required base band frequency is 100Hz - 5MHz can I use the AD8347 for this frequency band ? [AE] Yes 3. Base Band LPF: On page 17 of the data sheet capacitors C2, C4 are tied to VRef but on page 18 the LPF capacitors are tied to GND. What is the correct way to connect the base band LPF ? Answer There are two specifications of interest relating to your question. The -3db bandwidth of the quadrature demodulator stage, from RFIP/RFIN to IMXO/QMXO is 90MHz. There are two uncommitted baseband amplifiers on the AD8347, whose -3dB bandwidth is 65MHz. Whether you use the baseband amplifiers or not, the devices can easily accommodate your desired 100Hz to 5MHz bandwidth. It is relatively unimportant where the capacitors are terminated to, since VREF is at DC and is therefore an AC ground: the critical thing is that the TERMINATION RESISTOR is connected to VREF, so that the dc bias level at the amplifier input is correct. In figure 8, RL is connected between IAIN and VREF. In figure 11, R39 is connected between IAIN and VREF. So there is no real contradiction between figure 8 and figure 11. In general, I would recommend connecting the capacitors to ground, unless there is a specific reason to connect them elsewhere, because the ground plane can be relied on to have a very low impedance at high frequencies; this is not necessarily the case for an output pin of the AD8347.

Only Q path used in AD8347

Question I want to use AD8347 with its AGC function. And I wonder whether I could use AD8347 with single Q-path and AGC function. Could AD8347 work normally? Answer I would recommend you configure the AD8347 as shown in Figure 46 of datasheet for operation of the VGA in AGC mode. Connect the two mixer outputs to Detector Input VDT1 and Detector Input VDT2. This will configure the device in normal operation mode where both mixers are operational and balanced. Data will be available on both the I and Q path and the customer can decide to use one or the other and just decouple the unused path to ground via a capacitor.

The output impedance of AD8347

Question I want to know the baseband output impedance of AD8347 which can help design the filter considering the impedance matching. And is it necessary to add an amplifier driver between the AD8347 and the AD9216? Could you please help us and give some comments on it? Answer The output impedance of AD8347 will depend on the application. If you use the output of mixer( without IF amplifier), the output impedance is only 3 Ohm, it is current output. If you use the output from the IF amplifier, the output impedance is 1M//3pF. I don’t know if you have blocking requirement in their system, if yes, please consider ADL5387 or ADL5382 for much better performance for receiver. The IIP3 of AD8347 is a bit poor. And the output capability is also small. The big advantage is high gain, but it really depends on the input signal, if the input signal is big, then the gain is useless, the chip will get saturated. If they choose ADL5387 or ADL5382, the baseband DGA can be AD8366. This part was designed just for zero IF application.

The response time of AD8347 in AGC mode

Question I want to know the response time of the power detection function of AD8347, I hope the AGC circuit could be done within 4uS. could you provide any information about this? Answer The response time of the AGC is determined by the cap on pin 19, AGC pin. If you want a faster settling time, then you would use a smaller cap value.

AD8347 Demodulator Reference

Question The Base Band inputs IAIN, QAIN and detectors VDT1/VDT2 should be biased with Vref (1V). When biasing these pins from another source which is also ~1V, I see a failure in the AGC loop, it gets stuck at the minimum gain (VAGC = 3V). What could be the cause of this strange behavior? Answer The internal VREF is very well controlled and your applied voltage can never be as quite or clean as the chip’s band gap voltage. The mixer AGC output level is only 24 mVppk therefore if there is any noise or offset on the your applied voltage then the AGC will be set to the minimum as you are observing. Refer to Figure 34. You will need to use the chip’s internal VREF for the application to work properly.

What is the AGC loop settling time of the AD8347 for a pulsed RF input?

Q. What is the AGC loop settling time of the AD8347 for a pulsed RF input? ----------------------------------------------------------------------------------------------------------------------- A. A step input was applied to the RF ports and the amount of time it took the VGA to settle to its final value was recorded. The response time is dependent on the capacitor value on the VAGC pin. CAP VALUE RESPONSE TIME 1uF 50 usec 0.1uF 15 usec 0.01 uF 2 usec Attachments: VGA_ResponseTime.pdf