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Fully Differential Amplifier FOR AFE7222

eli
Guru 13485 points
Other Parts Discussed in Thread: AFE7222, AFE7225, THS4522, LMH6554, THS770006, LMH6555, LMH6551, LMP8350
I looking for dual fully Differential Amplifier to drive the AFE7222,AFE7225. The source output requirements are: minimum output load 10kohm & maximum capacitance 8pF (max2828/2829). ( it should replace AD8138) 
  • 0
    TI__Mastermind 22825 points

    THS4522 is a dual FDA with rail to rail output swing and its input(s) can be DC coupled. That works well with the AFE7222, AFE7225 analog input Common mode voltage of 0.95V and 2Vpp full scale which requires swings to 0.45V above V- (ground). However, THS4522's upper frequency limit of 145MHz may be short of the AFE  at ~500MHz (depending on the application requirement).

    Thanks,

    Hooman

  • 0 in reply to Hooman Hashemi
    Guru 13485 points

    Hello Hooman,

    See below customer feedback/question:

    I already look on this item. We intend to work in the WIFI 802.11n which has BW of 20MHz or 40MHz. I'm concerned about the 2nd harmonic intermodulations ( distortions) at high frequencies. Other difference is in output current (verses AD8138) which may indicates on the 3md and 3rd harmonics problem .

    I need the IP2 to be less than 50dBc. can I simulate (test) this issue ? maybe By applying high BW signal to the ADC via this OP-amp and see if there is a problem ? I would appreciate your quick response .
     
    B.regards
    Eli
  • 0 in reply to eli
    TI__Mastermind 22825 points

    Hi Eli,

    THS4522's supply current is far less than AD8138's (1.1mA vs. 17mA). So, it won't have the distortion performance of the AD8138 (which you have alluded too as well)! THS4522 Figure 7 shows distortion beyond 10MHz falls below your required -50dBc.

    In my opinion, if your system allows a negative power supply (e.g. -1V) to be used with your Fully Differential Amplifier (FDA), instead of operating single supply where V- is normally tied to ground, that alleviates the constraint of swinging too close to the V- rail and a whole bunch of devices would fit your needs! Your distortion numbers should also improve greatly as your output would be more "centered".

    The next thing that constraints your choices is that you want to have a dual device. TI offers a far wider selection of single FDA's. If the two things above are attainable, I've listed some of the higher performing FDA's for your consideration:

    LMH6554 (2.8GHz)

    THS770006 (2.4GHz)

    Here is a complete list of FDA's:

    http://focus.ti.com/paramsearch/docs/parametricsearch.tsp?family=analog&familyId=1453&uiTemplateId=NODE_STRY_PGE_T

    Hope this helps. If you need help with biasing the FDA with a negative V- power for this purpose, please let me know and I'll send you some examples.

    Thanks,

    Hooman

  • 0 in reply to Hooman Hashemi
    Guru 13485 points

    Hi Hooman

    need some information regarding the LMH655X series ,

    intend to work with 3.3V single voltage supply . However there is no data regarding this voltage supply. I would appreciate your help with finding data such as : Output current , 2nd & 3rd harmonics distortion or IP2 & IP3 . the load is the AEF7225.

    B.regards

    Eli

  • 0 in reply to eli
    TI__Mastermind 22825 points

    Hi Eli,

    LMH6551 has an Electrical Characteristics (EC) table for 3.3V supply. LMH6555 is completely characterized at a supply of 3.3V. These are probably the only LMH655X FDA's which allow 3.3V single supply operation. LMH6551 distortion numbers are probably not good enough for this application but LMH6555 has a chance. I've attached < 500MHz distortion plots not found in the LMH6555 data sheet for your review.3716.LMH6555 Distortion 4_12_11.xlsx

    Thanks,

    Hooman

  • 0 in reply to Hooman Hashemi
    Guru 13485 points

    Hi Hooman,

    • From data sheet LMH6555 has input of 100 ohm Differential Input Resistance .
    •  However the MAX2829 which is the source for this OP-AMP has Output resistance requirements of 10 kohm minimum . LMH6551 is defined for    G = +1, VS = 3.3V, VCM = 1.65V, RF = RG = 365Ω, RL = 500Ω . What is the input AC resistance of the AFE7222/5 ?

    Thanks

    Eli

  • 0 in reply to eli
    TI__Mastermind 22825 points

    Hi Eli,

    From what I can gather about the MAX2829:

    • You are talking about using a FDA on the "LO leakage and sideband-detector outputs" which have a 10k ohm minimum differential impedance requirement at 1MHz bandwidth! High speed paths usually are not 10k impedance levels!
    • The BW is limited to 1MHz

    Regarding the LMH6555:

    That device is meant for 50ohm single ended (100ohm differential) input environments (high speed) and is probably not suitable for this application.

    May be a device like the LMP8350 with 480kohm input impedance, as shown below, is more appropriate for what you need (since the bandwidth is rather low).

    Here is the AFE7222 Analog input definition showing an input capacitance of 4pF:

    With LMH6551 specs shown below, you'll have output swing limitations to drive the AFE7222 as its output is limited to 0.9Vpp with VCM=1.65V (or 1.2V relative to ground) whereas AFE7222 needs 0.95V as shown above. Unless you can use a negative supply for the V- pin of the LMH6551?

    Why did you abandon my original recommendation of THS4522? I still think that's your best bet. From what I can gather, you're not dealing with 20MHz of bandwidth since the table above shows a 1MHz limitation! So, please clarify if I'm not on the right track?

    Hope all this helped.

    Thanks,

    Hooman