On 3 May 2007 at 1:48, Peter P. wrote:

> In continuation of this thread, I have hinted before that I have a
> clamp circuit that uses generic parts and works well. I have posted it
> before on this list (eliciting no reaction). It is hardly
> ground-breaking, and it works very well. It can be implemented in
> several variants, and a zero power version is possible (by adding a
> MOSFET like a 2N7000 in series with R1, controlled by a voltage
> monitor or by a microprocessor output).
> 
> The clamp current is about 15 times the mirror current as seen in the
> waveform graphics from a transient simulation. The simulation data is
> obvious from the schematic. The enlarged waveform shows one
> transition, with clamp current ~ +/- 50mA (!) (red trace), and clamp
> voltages of Vdd+95mV and Vss-50mV, with no overshoots and no other
> troubles (like low 3dB corner caused by high R2 - R2 is only 100R
> here).
> 
> Of course the circuit is no snake oil, and the quiescent consumption
> of 3.6 mA is high. Also, the second and other channels shown (CL2 etc)
> are an optimistical feature, since the bias circuit limits the total
> clamping capability. BUT by adding a better bias circuit, using a dual
> low power rail-to-rail opamp or similar, the quescent can be brought
> down well under 100uA and output capability increased significantly.
> 
> One 'bug': The circuit will make it hard for the input to be pulled to
> 0V and Vdd, supplying current when the input is nearer than 100mV to
> either rail. Thus f.ex. ratiometric AD inputs cannot be pulled close
> to the rails (LSB/MSB cannot be attained) unless a very low output
> impedance sensor circuit is used. It is safe to assume that the 'zero'
> clamp interference input voltage area is Vdd+0.1V ... Vss=0.1V.
> 
> comments are welcome (this circuit has been in use for aeons, I
> personally have used it for 18 years, in both the shown symmetrical
> version, and single-ended, with clamp to ground and other voltages as
> needed).

Hi Peter,

Very cool circuit. Tidy, low parts count, low cost, good performance, I like it. I guess 
it could work on 3.3V ok, make R1 approx 470R?

If I had seen it before (I don't think I have) then it would have escaped my attention 
because I didn't think I needed to clamp with great accuracy. A bit more experience 
now after trying to design with zeners, schottly diodes, etc, and a bit more detail 
from Microchip on the importance of the +/-0.3V abs max specs have shifted my 
thinking.

Thanks, Brent.

-- 
Brent Brown, Electronic Design Solutions
16 English Street, St Andrews,
Hamilton 3200, New Zealand
Ph: +64 7 849 0069
Fax: +64 7 849 0071
Cell: 027 433 4069
eMail:  brent.brown@clear.net.nz


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