Is an active probe possible with a FET input op amp like the AD4717?
Do you mean Ada4817-1? Yes, it is possible. Look at http://welecw2000a.sourceforge.net/docs/Hardware/Aktiver_Tastkopf_mit_OPA659.pdf
ada4817 can replace opa659 there.
Another useful link is http://www.mikrocontroller.net/topic/188227
Mashkov is correct that ADA4817-1 can be used as the FET amplifier of an active probe. It has a very low input capacitance in the range of pF that will result to high impedance over a broader bandwidth. The low input capacitance will also offer low loading thus using of longer leads can be used. It also has high bandwidth of 1GHz. Its high input resistance of 500Gohms is a very good isolation of the circuit under test and the oscilloscope.
Hope this helps.
Hello mashkov, hi Anna,
there are some challenges.
Spec of ADA4817 for GBW and BW is at Vout = 0.1Vpp (slew rate limitation)
Target is as below...
Operating temperature: -40 deg C to 125 deg C
(105 deg C qualified part should be sufficient, I guess this device is capable of 125 deg C - only limitation should be power dissipation and no customer requirement yet for 125 deg C)
Should be able to drive 100 ohm load - 50 ohm at scope input in series with 50 ohm series termination at output
Input impedance of 1M ohm at probe input (can reduce to 100k ohm at high frequencies, similar to active probes on market)
Target would be to have 10:1 probe - but even 100:1 should be okay.
Vin range of +/- 20V.
A unity gain setup (as with the OPA659) using the ADA4817 (see image) will not work.
The above circuit is unity gain - but limitation due to Vout swing, for example at -20V to +20V swing, the output will swing from -4V to +4V.
Please let me know if there are some solutions to this.
You need to use 10:1 divider at input to get +-20V input range, and add fast high output current op-amp such as AD8009 at G=+2 powered form +-6V supply (after ADA4817) at output. You need some kind of input protection also.
If you end up using the AD8009, be sure that the equivalent resistance feeding the non-inverting input is less than or equal to 150 ohms.
Can you advise the background please?
The AD8009 can latch up with source resistance greater than 150 ohms on its non-inverting input.
How about the idea of using the following together?
Input stage with AD4817
Output with the AD8000
Output voltage swing of AD4817 will be limited to 0.1Vpp for an input swing of 5Vpp.
AD8000 will be unity gain.
Input resistor divider ratio of 50:1 - 980k series with 20k divider - This will load the high frequency - right?
May be 50:1 ratio is not required. A 10:1 ratio is sufficient.
I saw the spec of the ZS1000 active probe from Lecroy.
Spec if +/-8V input range, 1Mohm/0.9pF input impedance, output impedance 50 ohm.
But assuming that most signal at such high frequencies are only harmonics contents of a digital square wave whose slew rates and jitter are important to be probed, it is a good enough assumption to consider a 0V to 5V digital signal swing at 1/5th the bandwidth of probe ~200MHz digital signal with 5th harmonic at 1GHz. Here from Fourier series expansion it is sufficient to have 1/5th the amplitude of the fundamental at the 5th harmonic. This means (1/5)*5Vpp signal has to be within the small signal bandwidth of the amplifier. So 1Vpp input is to have a only 0.1Vpp swing at the output of the AD4817. The large signal bandwidth at 5Vpp input is sufficient if it is at 200MHz.
So, a 10:1 input divider will work for most digital logic evaluation.
Experts, please guide me if my way if thinking is correct or wrong.
Any help is useful !
Single ended active oscilloscope probe like this http://www.home.agilent.com/en/pd-1912806-pn-N2795A/active-probe-1-ghz?nid=-34024.959292&cc=US&lc=eng have 10:1 attenuation and +-8V input range, so generally You are right. Due to SR limitation maximum AC amplitude start to drop at some frequency. Look at Voltage derating over frequency graph at page 3 of this http://www.home.agilent.com/agilent/redirector.jspx?action=ref&cname=AGILENT_EDITORIAL&ckey=1967534&lc=eng&cc=US&nfr=-34024.959292.00 paper.
Note that physical details of construction and PCB layout very important for such hi-frequency wideband circuit.
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