We are planning to use the ADA4817 1GHz Fast fet op-amp (amazing device that it is!). We would like to use it as a 2:1 multiplexor.
We note that it can be powered down (with Iq = 1mA), however there is no information on the output topology when powered down.
Assuming that we will have a series resistor between each output and a common summing point (which happens to be a ADA4939 differential amplifier), do you have any idea about the following points:
1. What is the load presented by the powered-down op-amp output (as in R//C) ?
2. If the output is a relatively low impedance (ie 100 - 1k), how variable is it from device to device and over temperature
2. What is the effective output noise (nV/rtHz would be good) for a powered down op-amp?
Thanks for any help.
Can you send me a schematics of how you plan to connect the ADA4817 with a series resistor between each output so that it looks like a ADA4939?
Can you send me an email at firstname.lastname@example.org?
It will be much easier to discuss there.
I did get a response from Analog Devices:
Please see attached the simulation from the designer. The short conclusion is that the ADA4817 is high impedance when off. It looks like several Megohms in parallel with 3.5pF, and that the ADA4817 doesn’t contribute much to high frequency system noise when off.
•ADA4817 set in a G=+1 configuration, Rf =200Ω, +-5V supplies. Tdegc =125°C . Disable voltage set to 0V
•Test AC current (magnitude =1) applied to output. This is a small signal test. Output voltage was then plotted.
•For “off” noise, the ada4817 was simulated for two cases, (feedback resistor) Ro=200, (gain resistor) Rg=1.1k, and Ro=200, Rg =147
•ADA4817 off output impedance is high impedance when off. It looks like several Megohms in parallel with 3.5pF.
•ADA4817 doesn’t contribute much to high frequency system noise when off. Simulation is predicting noticeable low frequency noise contribution from the amplifier that scales with the resistance of the feedback network.
Message was edited by: Bart Schroder, 14 June 13 We built this multiplexor, and discovered that the output portion of the multiplexor works as expected, which is great, but sadly that there is a problem with ADA4817 silicon which greatly reduces the usefulness of the part. The JFET input is incorrectly biased during shutdown, which effectively connects the Input + to a diode to -VEE via a resistance of about 7k ohm. Thus what was a high impedance input becomes a low impedance. This trashes the use of the device in high impedance dual op-amp multiplexed situations, as the device that's off pulls the input of the device that's on to -VEE. Be aware of this if using the part.
Hello, I asked the original question. We built the system, and found the following:
1. When the PD is activated the ADA4817 output goes to -V with what appears to be an approx. 600uA current sink. As the output voltage is varied by the other amp in the multiplexor the current varies. This reduces linearity a lot, and renders the multiplexor pretty much useless.
2. When the PD is activated the ADA4817 input goes to V- as previously noted with an approx. 9K resistor. This means the input, if attached to a node common with the other amps input, will affect the other input.
So effectively don't use the ADA4817 as a multiplexer. It is a pretty good amplifier otherwise. We got published distortion and noise out of it.