How can I drive the shutdown of this opamp + analog switch? - Page 1
Forum section: electronics / projects

author: Verdefluox
author_rank: Contributor
timestamp: December 13, 2024, 11:31:46 am
content: Hi folks,I am currently building a wideband amplifier, which has included optionals active low pass filters.The configuration is the following: I have a SP3T front switch (technically, two SPDT arranged as such, with 3 states possible), which has to drive the three band options. Every channel, when selected, has to enable BOTH an analog switch AND the opamps for the filter (NOTE: the wideband only has to drive a switch, and the 200 Hz LP has the same configuration as the 20k Hz, so they are not shown for simplicity purpouses). All the components are powered by the +-7V rails, and the logic is a 5V one.I am wondering if the proposed solution can work: the switch is powered directly by the 5V logic after the front switch (so either 5V or 0V), and the opamps are powered/or not by a comparator, which also does directly the level shifting from 5V to +- 7V. I am doing this to save power when I am not using the filters, since the whole system is powered by batteries. The 2V5 can be easily obtained by a resistor divider from the 5V, for example.Problem is: the datasheet for the opamp does not state directly the maximum levels allowed for the SHDN pin, and it simply states "it's best to leave it floating" (how much performance degradation can I expect otherwise?). I am assuming it works as usual if I keep the SHDN pin powered (in one way or another WITHIN THE RAILS), but I'm not sure about this. Note also that the graph only tells me the SHDN current up to 5V above V-, so I don't know if I'm allowed to go beyond.I have attached what's written in the datasheet, and I'll link it below, since maybe I missed something on the way.ADA4622 datasheet:https://www.analog.com/media/en/technical-documentation/data-sheets/ada4622-1-4622-2-4622-4.pdf

author: Andree Henkel
author_rank: Regular Contributor
timestamp: December 13, 2024, 01:12:08 pm
content: I don´t think there will be any trouble as long as your shutdown input voltage is not above the positive supply of the OPA, so in your case 7V. You probably may get some unwanted current into the shutdown pin, but shut not be more than a few µA.If test shows otherwise, or you are really scared add a "digital Transistor" and tie the Collector to the Shutdown pin, base to output of TLV OPA and Emmiter to -7Vlikehttps://www.digikey.de/de/products/detail/nexperia-usa-inc/PDTC114ET-215/1157277

author: SCSKITS
author_rank: Regular Contributor
timestamp: December 13, 2024, 01:51:20 pm
content: The op-amp disable pin needs to be within 1.4V of the (-) supply to be disabled. You can not easily do this with logic levels or with a simple transistor circuit driven from logic. You might want to look at using a optocouplers for the op-amp disable control. They can easily be controlled by logic.

author: Verdefluox
author_rank: Contributor
timestamp: December 13, 2024, 02:20:16 pm
content: QuoteI'm sorry, but this is why I'm using the comparator. Why shouldn't it work? Its outputs are +-7V in this configurations, so -7V can be reached.

author: Andree Henkel
author_rank: Regular Contributor
timestamp: December 13, 2024, 02:27:48 pm
content: @SCSKITSHis circuit:Operating from +/-7V the TLV3702 Comparator with push-pull output should give a "low" output close enough to -7V.What I posted didn´t suggest a "simple" transistor, it will work driven by the Comparator Output and will provide an open collector output to control the shutdown pin, and because the emitter is at -7V it is able to control the ADA4622-1

author: Terry Bites
author_rank: Super Contributor
timestamp: December 16, 2024, 05:35:08 pm
content: I wonder how the amp in shutdown looks to the external circuit.It won't pass the signal but it doesn't disconnect it either. Could it become a source of distortion/ noise?Dunno, would be interested to hear about that.

author: jwet
author_rank: Frequent Contributor
timestamp: December 17, 2024, 05:50:42 pm
content: It will be fine- this is what shutdowns do, they debias the part but allow it to stay in circuit.Another suggestion, the DG419 have the equivalent of your level shifter/comparator built in- that's what's behind the Vl pin. Just put your shut down into the switch with rails tied to gnd and -7 or +-7 and Vl to 5v and you'll get it all.