switch between 2 voltages to bias a circuit

[crunx]

I have worked a lot with OpAmp designs, and based on my experience I would do a few things slightly differently.

I.M.O. The first thing I would claim is that MAX419 is not suitable at all for this application. It is simply too slow, with both limited bandwidth and low slew rate. Also, it doesn't work well with gains under 10, but may have stability problems.

Therefore you should check, that the amplifier is fully specified and performing properly when fed with the available voltage (5V, I suppose in this case).

Some things to look at:

You should definitely select an amplifier with sufficient gain-bandwidth product. Likely around or a little over ten MHz would be good enough for descent performance here. Another important parameter is the slew rate, which defines the maximum speed of large amplitude. For 3V in 1 us the theoretical limit is 3V/us, but in practice the amplifier should be faster - especially because the condition used for specification is likely easier than in the real application with some output capacitance etc. That also should reduce overshoot and ringing, as a slow amplifier would not react to its own feedback fast enough to swing nicely.

Also, if you wan to use the OpAmp with low gain such as a G=1 buffer configuration, check that it is guaranteed to be stable in that condition ("unity-gain-stable" is required for buffer / G=1 situation), and also tolerates your output-loading capacitance.

Ral-to-Rail I/O functionality is good, as then it is easier to get everything working right. (No clipping, and a good performance in the full supply voltage range)

For instance, MAX44259AUK in SOT-23 package likely would work with capacitive loads up to around 200-250pF. I have earlier proposed in this message chain some other amplifiers as well, at least one from Texas Instruments with even better capacitive drive ability. To find good enough OpAmp should be relatively easy, as there are quite many on the market performing well enough for this kind of application.

[TucoRamirez]

I have worked a lot with OpAmp designs, and based on my experience I would do a few things slightly differently.

I.M.O. The first thing I would claim is that MAX419 is not suitable at all for this application. It is simply too slow, with both limited bandwidth and low slew rate. Also, it doesn't work well with gains under 10, but may have stability problems.

Therefore you should check, that the amplifier is fully specified and performing properly when fed with the available voltage (5V, I suppose in this case).

Some things to look at:

You should definitely select an amplifier with sufficient gain-bandwidth product. Likely around or a little over ten MHz would be good enough for descent performance here. Another important parameter is the slew rate, which defines the maximum speed of large amplitude. For 3V in 1 us the theoretical limit is 3V/us, but in practice the amplifier should be faster - especially because the condition used for specification is likely easier than in the real application with some output capacitance etc. That also should reduce overshoot and ringing, as a slow amplifier would not react to its own feedback fast enough to swing nicely.

Also, if you wan to use the OpAmp with low gain such as a G=1 buffer configuration, check that it is guaranteed to be stable in that condition ("unity-gain-stable" is required for buffer / G=1 situation), and also tolerates your output-loading capacitance.

Ral-to-Rail I/O functionality is good, as then it is easier to get everything working right. (No clipping, and a good performance in the full supply voltage range)

For instance, MAX44259AUK in SOT-23 package likely would work with capacitive loads up to around 200-250pF. I have earlier proposed in this message chain some other amplifiers as well, at least one from Texas Instruments with even better capacitive drive ability. To find good enough OpAmp should be relatively easy, as there are quite many on the market performing well enough for this kind of application.

hello, the device  is the DG419, not the max419

Posted on: December 12, 2015, 11:25:37 23:25 - Automerged

Did you mean the DG419 works fine at5V? MAX 419 is a very slow Op-Amp.
The spikes are probably the result of a feedback effect due to the rising voltage or just crosstalk from your gate to the output.
It will probably go away when you put a load on the output, even if its only some kohm
But what the loaded curve shows: the analog switch has to much resistance to charge your input capacitance in less than 1us.
You need to operate it with a higher voltage (12V), that will decrease the resistance or add an OP-Amp as a buffer.

@vern
there's no max419 involved .  Anyway i'll increase Vdd to 9-10V on monday and check how much Rdson i can reduce.  the 'load to the output' concerns the logic gate clock? 

[vern]

TucoRamirez,
not a load at the gate clock, but at the output of your analog switch.
The spikes should not bother you anyway because you operate your device always with the load?
If the spikes bother you you could add a small capacitor to the output of your gate clock, that will slow down the rise and fall time of the clock.
You have to experiment with the value, I would start with a 100pF and see what the effect is.
That might dampen the spikes.
But I would not bother with it because the output voltages with the load look good, it just has a slow rise and fall time.

[crunx]

I am sorry about misunderstanding and confusion.

DG419 is indeed a SPDT switch. On data sheet the supply voltage range is specified to be " Single-Supply Operation +10V to +30V, Bipolar-Supply Operation ±4.5V to ±20V". Therefore on single supply 10V is the minimum for operation according to the specification. However, the part seems to be optimized for higher supply voltages, and the ON resistance is not fully specified on 10V, but on +/-10V. The data sheet has only a "typical" performance curve for 10V total feed, presented for +/-5V supply on page 5, the curve hovering around 35-45 ohms.

Therefore, be aware, that a switch optimized for lower voltages would be performing better. But when fed with at least 10V DG419 should work within specification - if you don't have too much load capacitance for the relatively high ON-resistance.

Also, the control voltage on "D" input of DG419 should go (nearly) to the supply voltage, otherwise the switch will not work right. -> Your switch-controlling oscillator should output at least 8 volts, when the supply is 10V!

Another mechanism causing glitches is charge injection.  It means simply said, that some of the control voltage will be coupled to the switch output. That is especially visible during those few ten nanoseconds, when both NO and NC sides are disconnected during change of state (switching off is slightly faster than switching on). I have "admired" that phenomena in some of my own designs, and it can be a real nuisance!

[TucoRamirez]

have u seen the schemas on post #17 ?   anyways i'll try to quantify tomorrow the Rdson approx value applying some capacitive loads and checking Tau...

[crunx]

I have seen the drawing you presented.

On the schematics you specify +5V as being the feed voltage. Your inverter-oscillator is likely fed from the same supply? If so, then rising the feed voltage of "everything" should satisfy the requirement of sufficient drive amplitude for the analog switch. And you are limited first by the inverter's maximum supply voltage (15V).

In any case, if I were you, I would prefer to keep the 5V operation, and implement this kind of design with another analog switch, which is designed to work well on 5V supply. I made a quick check at Maxim's data, and found, that MAX4544 might be more suitable. It's specified for 2.7 to 12V supply, and has maximum on-resistance of 60 ohms. If the maximum resistance would be a problem, there are also other switches with only a few ohms of ON-resistance. However, the charge injection issue is usually larger for low resistance switches, as the MOSFET transistors inside the switch are in that case of larger size/capacitance.