Voltage Gain Switch Troubleshooting

Hi yourself.

If you recall Win Hill died about two months ago but I suppose that since my newsreader shows 57034 messages and I don't know who the f*ck you are it is quite likely that, in as much as I don't know who the f*ck you are, you missed the obituary as well.

Having said that I notice that your description of the circuit you are implementing seems to differ from the one that the now Defunct Win Hill suggested in the first place.

Although he may now be a dead bloke there is a good chance that you have f***ed something up and I really don't think dead people are sort of bothered about sorting out your f*ck ups.

'Thanks in advance' doesn't really work now, does it?

DNA

Genome ha escrito:

I`m sorry to hear that. I didn`t have a clue, I try to check the group once in a while, but I missed that. There's thousands of posts as you pointed out.

I assure you that the fact that I quoted Win is because his schematic was convenient for illustrating my circuit, nothing more.

I`ve never met Win personally, but he's always been very kind and helpful. I shall remember him through what I`ve read from him, and that's no small thing.

Again I`m sorry, but I guess that like "Thanks in Advance" this doesn`t work.

On 19 Dec 2006 15:27:53 -0800, snipped-for-privacy@gmail.com wrote in Msg.

[...]

Don't let Genome pull your leg *too* hard. Win Hill is alive and kicking unless it was an impostor who posted something only yesterday on this newgroup, imitating his writing style.

But, like so often, it is a Genome post that can point your thinking in the right direction. In this case it's that Win isn't getting younger, and we're still holding our breath for AoE 3rd ed.

robert

Robert Latest ha escrito:

Well, I guess that explains why I couldn`t find any Winfiel Hill obituary post. Genome got me 100% convinced, I`ll leave my eyes open from now on.

Back on topic. Looking back at Win's original post he states that minimum gain is 2, and I used a gain of 1 without feedback resistor. Also the stage 2 gains that I stated were 100 and 500 are in fact 10 and 50, sorry for that.

I re-assembled the ckt in a protoboard and found out that gain of 1 is a no go, but if I put a feedback resistor of 100k the circuit works normally. Also if I use other gains like 2, 5 or 10 they work, but I`ve noticed that as the gain-setting resistors become smaller the output swing also becomes smaller. The PCB ckt remains unchanged, could it be that the gain of 1 path is affecting the other path's output swing? I mean they should be isolated and independent, but that's my only guess. I don`t wanna cut the track and later find out that it didn`t work.

Can someone explain why the gain of 1 config. doesn`t work with the

4051 on the feeback path ?

snipped-for-privacy@gmail.com a écrit :

because the 4051 will clamp the opamp output signal to +/-(5V+Vd) with its internal... clamp diodes.

--
Thanks,
Fred.

Fred Bartoli ha escrito:

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Ok thanks, I think that's it. The unity-gain path is clamping the output regardless of the feedback path selected. I`ll cut the track and add a resistor. Problem solved :-)

I doubt that'll solve the problem. Normally we have to use high-voltage cmos switches powered from the supply rails for general purpose full-swing circuits, but my post suggested a hack allowing the use of cheaper low-voltage logic-family switches, provided your gain was at least 2x. Viz,

"Gain switching the feedback dividers eliminates effects . __ from the CMOS switch's series resistance. . ------|+ \\ . | >-----+----+----+--- . .--|-_/ R1 | | . | | R3 | non-inverting gain . '---o-->o----+ | R5 . o--- | ---+ | . o--- | -- | ---+ . | | | . R2 R4 R6 . | | | . GND --+----+----'

And if Gmin = 2, i.e., R1 => R2, etc., then a +/-5V switch can be used with +/-10V output signals."

I don't see how you can cut a trace and add a resistor and thereby magically get around this principle for G = 1.

But there is a trick you can use to get full output swing with half-swing capable switches such as the hc4041 family: first attenuate the signal by 2x, and follow with a gain of 2x for the G=1 overall-gain position, etc., and so on.

--
 Thanks,
    - Win

Winfield Hill ha escrito:

I have found different results, I tested the following circuit:

From Stage 1 |\\ To voltage follower o--------|+\\ | >--------------o-----o-o .-----|-/ | | | |/ | | | | .-. | | | | | 100k | | | 100k | ___ | '-' | o-----|___|---' | '-----o--__ | o-------------------o | TC4051BP | .-. | | 100k | | '-' | | === GND (created by AACircuit v1.28.6 beta 04/19/05

The output is able to swing fully with this configuration, regardless of the switch's current position. Are there any problems that I`m not accounting for ?

I wouldn`t have had any problems had I not overlooked the Gmin=2. If this workaround doesn`t work I`ll have to redesign the whole thing.

In the top position, ie, gain = 1, have you driven the input with a sine or ramp and used a scope to check that the output is gain = 1 and that it's following the input and not just running out of feedback loop once the output reaches the switch supply voltage ?...

Chris

ChrisQuayle ha escrito:

You are right, I tried the circuit in Pspice and it does the strangest thing. This made me realise that the node of the inv input has to be able to allow 10v for the loop to work correctly, and that's unbearable for the switch.

So I guess I`ll reduce the first stage gain to 50, then followed by second stage gains of 2, 10, 20 and 100, to produce 4 different gains of 100, 500, 1000 and 5000.

That PCB's going to waste, back at the start now. Can I call it problem solved now ?

It's still not an optimum design, since the circuit should be designed so that device ratings are not exceeded under *any* conditions, not just those expected in normal operation. While the switch may withstand being over driven, it still leaves a corner for failure which is not covered by the device data sheet.

Two possible approaches:

1) Put in a switch that you can run from the op amp supply. The 4051 is a very old device (1976 ?) now and there are much better devices available.

2) Run the op amp from the +/- switch supply

A third way may be to attenuate the output before applying to the switch, say 10-1, which brings the switch input well within range, but this is a more complicated solution. Also, the minimum gain then becomes 10.

Might seem a little obsessive, but I never feel happy unless a circuit design looks ok from all angles. Do it right to start with and you never need to look at it again etc :-)...

Chris

Whoaaaaaaaaa!!!!!! FREAKY.

You're meant to spin in your grave not claw your way out of it and answer the question.

DNA