Re: Voltage Gain Switch (Design Help)


Phil Hobbs ha escrito:

leo2100@xxxxxxxxx wrote:
Phil Hobbs ha escrito:
(snip)

I might use a voltage divider something like this:

0-----RRRRRRR------*--------*-------*-------0
| | |
30k R R R
R R R
7k50 R 3.33k R 610 R
R R R
R R R
| | |
0 0 0

2N7002 2N7002 2N7002

gnd gnd gnd

You could probably get away with some DMOS peripheral driver chip for
all 3 switches, because none of them will ever get further than 0.2V
from ground in normal operation, so none of the protection diodes will
turn on. The 2N7002 gets down to 2 or 3 ohms on resistance, with some
very small nonlinearity at the current levels we're talking about.
Their capacitances are in the 100-pF range at low voltage, giving a 3 dB
frequency of something like 18 kHz for three of them.

I`m going to answer the designs one by one.

About this one, I understood the general idea, which I believe is to
switch gains through the MOSFETs. But, I have no idea about the second
stage, the attenuator. First the signal gets amplified then attenuated,
the overall gain of the two stages being at least 2. Then it goes to
the third stage which has a fixed gain of 50 ? Which of the stages has
the selectable gain (voltage divider)? You probably should mention a
standard DMOS driver because I have never used one before.

It`s unlikely that I will use this design, but I am interested in how
it works, seems rather unconventional :)


Well, unconventional in the sense of being a minor variant of the one
used in every oscilloscope in the universe, at least until recently....

You're right about the nonlinearity and on-resistance problems with
switches--which is a a strong reason to keep the amplifier gains fixed,
and do the gain switching at low-to-moderate signal levels. There are
also pickup and noise peaking problems with hanging all sorts of
gingerbread on summing junctions. Your predecessor used relays to
switch fixed-gain amplifiers in and out, which isn't a bad design for a
one-off.

What's so hard to understand about the design I posted? You need a gain
of 100, 500, 1000, or 5000. Taking out a common factor of 100 for the
first stage gain means that the amplifier noise, offset, and drift
behaviour will basically depend only on the first stage. This is an
important point in instrument design.

A passive attenuator using big honking MOSFETs (2 or 3 ohms ON
resistance) gets rid of the switch nonlinearity completely. The ones I
show cost 20 cents each, and are available everywhere including
Antarctica. Putting the switched attenuator between the two amps means
that it can run at the same narrow range of signals at all times, which
makes doubly sure that the MOSFETs are not going to cause a linearity
problem. That's actually one of the nicer features of this circuit.
Choosing a second-stage gain of 50 means a gain of 1.00 for the
attenuator's highest gain setting, which is easy to do accurately.

The output amp, with a gain of 50, gets you back up from +-200 mV to +-
10V. You don't need switches with a huge voltage range, and the
attendant resistance nonlinearity and temperature drift. *Note that you
can't tune out the nonlinearity and drift with a pot.*

Once again, this design doesn't cause excess noise, offset, or drift,
because the overall gain up to the attenuator output is always at least
2. This is simple to prove--about 3 lines of algebra. It's simple,
cheap, made from jellybean parts, and will work very well. Other than
that, it stinks.

Cheers,

Phil Hobbs

Well, I understand now, and I must say I really like the idea. It`s
kind of unconventional in the sense that you are the only one among the
one's who replied that proposed this idea instead of the analog switch
or relays ;)

The only problem is that the control signal comes in binary (from a
PIC), so I will have to use a decoder to drive the MOSFETs accordingly.
That adds another IC to the circuit, yet another thing that can go
wrong (but very unlikely). The decoder pops a question: what VGS
voltage shoul I use ?. From what I gather, more VGS voltage better
linearity and for a wider range of VDS. I was thinking about VGS=10V,
but I will need to raise up the voltage coming from the PIC with a BJT,
so I think VGS=5V is fine, the VDS voltage is already low enough so it
doesn`t matter.

.

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