LMH6518 - for Scope Application

Hello Anbu,

Input CM voltage: The LMH6518 input CM voltage should be close to Vcc / 2. So, if you are using a single +5V supply for the LMH6518, the CM should be around 2.5V (0.5V CM would not work). The range is from 1.9V to 3.1V or +/-0.6V around the mid-rail:

Maximum Output Voltage: The LMH6518 output linearity will not be very good if output swing is much beyond 800mVpp. From this plot, you see that the output vs. input plot looses linearity if y-axis is more than 1400mV or lower than 1000mV (corresponding to 0.4Vpeak or 0.8Vpp output):

Input Voltage Range: The 1Vpp differential input condition you have noted is on the "very edge" if you look at the typical plot below (which corresponds to the lowest LMH6518 gain, being LG, 20dB). I'm referring to the x-axis traversing +/-0.5V where the plot is still fairly linear:

So, yes it is doable but you do not have much margin left. It would be better if you design your front end attenuator for more margin in this regard. Also, if your input CM is not exactly centered, your margin might be even less!

There is an Excel spreadsheet in the LMH6518 product folder which may help:

http://www.ti.com/tool/lmh6518design-calc

Regards,

Hooman

Hello Hooman,

Thanks lot for your advice..

1. ) OK ,  i will set input CM = Vcc/2 ,  2.5 Volt for 5 V supply  as recommaned my datasheets.

2. )In my project we using  Intersil (ISLA118P50) - 500Mpbs ADC for Piggy module, which support

    Full scale Input  Vin = 0.725Vpeak , 1.45 Vpp and Vcm = 0.535V   

    LMH6518 max out 0.8Vpp half of my ADC FS input, so shall i use one more Diff Amplifer  with Gain 2 inbetween LMH6518 and ADC input ?

Pls advice me ..

Regards

Anbu M

Hi Anbu,

The LMH6518 has internal 50ohm resistors in series with each output. So, with 500ohm diff input impedance of your ADC, you can expect a full scale ADC input of 1.33Vpp {=1.6Vpp * 500ohm / (500ohm + 100) }. The LMH6518 0.8Vpp maximum output spec is with an ADC diff input impedance of 100ohm.

However, the 0.535V VCM of your ADC is something that the LMH6518 output CM cannot handle directly. The LMH6518 CM range is 0.95V to 1.45V, as shown below, unless you bias the LMH6518 with unconventional supply voltages (i.e. tie the device ground to a negative supply voltage):

I recommend using a fully differential amplifier after the LMH6518. The following arrangement can meet the ADC CM and FS analog input conditions. I will use the LMH6554 as an example:

V+= 3V, V-= -2V, VCM_input tied to 0.53V

Done this way, you get the proper swing and you would be able to achieve the 0.53V CM condition of your ADC.

Regards,

Hooman

Hello Hooman,

OK ..Thanks lot .. 

Wishing u "Happy New year"

Regards

Anbu M