AD8367 as an AGC on a 650KHz - 750KHz IF, DC bias and output range question

Hello,

I have a design with a tuner providing a 650KHz to 750KHz IF which is then sampled by an MCU at 1.6MSPS. The MCU has single end ADC input and the tuner has a differential output of up to 2.0V P-P amplitude.

In the first iteration of this design the tuner IF feeds a 4:1 impedance transformer used as balun, the unbalanced output of the transformer goes to an OPA365 for scaling and adding1.65V DC bias then it goes to the ADC input. 

This solution is working but the transformer is rather large and the room is sparse on this design (USB dongle form factor), also the OPA365 gets easily saturated and introduces asymetrical distortion (the bottom tends to clip and the top tends to fold back and make unwanted product), also there is the problem that the IF amplitude varies too much which decrease the effective range sine the signal processing chain can only handle 3 bit of resolution loss (on 12bit).

Also later on i discovered that if i grab the IF signal on one side of the differential output before the transformer there is alreadya 1.65V DC bias, the amplitude is pretty good and the effective range is the same as if i use the IF at the output of the OPA365, with the advantage that the clipping is nice (it is the tuner clipping, which is clean and symetrical) and the noise floor is lower.

So i want to get rid of the transformer and OPA and add an AGC on the IF to keep it level constant as long as possible, in order to stay in the best range of my ADC. My idea is to grab the signal where i currently take it, directly on one of the differential lines of the tuner, and feed it to the AD8367 directly, which would be configured as an AGC and would hoepfully keep my ADC with high signal level even when the IF at the tuner output gets lower.

HEre are my question in this regard:

-will the AD8367 accept an IF that is already DC biased at 1.65V?

-does the AD8367 offer any option to introduce a DC bias at the output?

-will the AD8367 provide sufficient output level for my ADC (current range 0 to 3.3V, if needed i could possibly lower the VCC and thus the VDDA, which must be kept close on this MCU)

Thank you in advance.



correction
[edited by: Hoel at 5:41 PM (GMT 0) on 9 Apr 2020]
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    •  Analog Employees 
    on Apr 13, 2020 9:15 PM

    Hello,

    As commented on "INPUT AND OUTPUT INTERFACES" section, ac coupling is needed in case where an elevated signal is used. So suggest putting ac coupling(dc block) capacitor at AD8367 Input. And make sure the signal level on AD8367 input from tuner is not as big as 700mVp_p to avoid overloading. Input impedance is 200 ohm nominally and so you may need the reactive matching network to Tuner if Tuner is at different output impedance. Output impedance of AD8367 is 50ohm but it operates well in a 200 ohm load. All data on datasheet are with 200 ohm input/output system. Examples to interface to 50 ohm are shown at Table5.  If your system is at true 50 ohm for Tuner output or ADC input in MCU, take a look at AD8368(less gain tunable range).

    The output center voltage(common mode voltage) is normally set at Vs/2, and says it can be adjusted by applying a voltage to DECL. 

    Gain range of AD8367 is upto 42.5dB. It will depend on Tuner output level, however assuming 200 ohm source and load case as shown at Fig42 , 20mVp_p at input will be 3.3Vp_p at output and to ADC input.

    Thanks

    Tony

  • Hello,

    Thank you for the return, 

     If your system is at true 50 ohm for Tuner output or ADC input in MCU, take a look at AD8368(less gain tunable range).

    The system is definitely not 50Ohm, the tuner output is 2.2KOhm but it can drive 200Ohm at 2.0V p-p, i had it feeding a 200Ohm 4:1 impedance transformer / balun on a previous version of this design. The ADC is high impedance. I added provision for DC blocking cap at the tuner output and for matching networks on both side of the AD8367.

    as shown at Fig42 , 20mVp_p at input will be 3.3Vp_p at output and to ADC input.

    As for actual output level, i have seen a transfer curve with 20dB gain if the input is below -35dBm and constant  -15dBm output if input is over -35dBm (up to +15dBm). This was in AGC - negative slope mode.  I hope i can expect 3.3v p-p output in AGC - positive slope mode.

    The tuner has a VGA so i can set the maximum output anywhere up tp 2.0V p-p.

Reply
  • Hello,

    Thank you for the return, 

     If your system is at true 50 ohm for Tuner output or ADC input in MCU, take a look at AD8368(less gain tunable range).

    The system is definitely not 50Ohm, the tuner output is 2.2KOhm but it can drive 200Ohm at 2.0V p-p, i had it feeding a 200Ohm 4:1 impedance transformer / balun on a previous version of this design. The ADC is high impedance. I added provision for DC blocking cap at the tuner output and for matching networks on both side of the AD8367.

    as shown at Fig42 , 20mVp_p at input will be 3.3Vp_p at output and to ADC input.

    As for actual output level, i have seen a transfer curve with 20dB gain if the input is below -35dBm and constant  -15dBm output if input is over -35dBm (up to +15dBm). This was in AGC - negative slope mode.  I hope i can expect 3.3v p-p output in AGC - positive slope mode.

    The tuner has a VGA so i can set the maximum output anywhere up tp 2.0V p-p.

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