Voltage Clamping/Regulation. 1

[morizabal]

I am currently designed a simple instrumentation amplifier "INA125" from Burr-Brown. I am powering with a 7662 charge pump to provide a bi-polar voltage. My design specification is that the amplifier operate from 12-24 VDC. The INA125 operates to a max of +/-18 VDC and the charge pump to +/- 15 VDC. Basically I want to limit the voltage to power the INA and charge pump to 12VDC. I want to be able to operate the INA to produce a 0-10 VDC output. But in order to get a 10 VDC output the INA requires a minimum of at least 11.5 VDC. I tried using an 12 V Zener Diode and Resistor but found the volt drops below the required when I apply the load. If I drop the protection resistor too much, it over heats the Diode, and I don't want to use a high wattage diode. I also tried using a 12V regulator, but that required at least a 13 volt input to generate a 12 volt signal. What would be the best way to take care of this problem. I need to be able to limit the voltage to these chips to prevent damage.

 

[nbucska]

The INA needs +11.2 and -10.3 V. Use 7662 to get negative
12-24 V and use LDO (low drop-out ) regulators e.g.
from <

http://www.maxim-ic>.

They have both + and - LDO-s. <nbucska@pcperipherals.com>

 

[morizabal]

What I need is to regulate a voltage of at least 11.2 VDC at 12-24 VDC input. My regulator needs to source at least 100mA of current. I have looked at the LM2931 from Nat. Semi. which meets these specifications. I would like to use fixed resistors to adjust the regulation voltage rather than pots. The LM2931 has a drop out of .3 volts, but I am afraid that fixed resistors even at 0.1% tolerance would give me too much of a variance when I add the tolerance of the LM2931. I would get a range of 10.77 to 11.95. The lower number is too low and the higher number would require at least 12.3 VDC Input. Are there any other devices that might accomplish the same task. I am trying to keep parts also at a minimum. Is a pot probably my best option?

 

[nbucska]

Evidently there are more requirements/complications not
mentioned in your original posting.

The INA125 or the 7662 can't supply 100mA !


What are you using the INA output for ?( ADC, etc.)
What is your signal frequency range ? How much accuracy
do you really need ? Other requirements ?

What do you need 100mA for ? could you reduce the +/- 10V ?
How many circuits do you need ? Is price/space/power critical?

Is the V+ input stable? Could you afford to build
a discrete regulator with pass-FET ?

what is the spectrum/waveform of the signal like ?
You may exploit the good power supply rejection
of the INA to use a swinging supply !




<nbucska@pcperipherals.com>

 

[morizabal]

I need the 100mA to supply power to the INA125, 7662 and the sensor. The sensors are bi-directional strain gage based force sensors with a 2-3 mV/V sensitivity. Our mininum bridge resistance is 350 Ohms at either 5 or 10 Volt excitation, and our desired outputs are between 0-5, and 0 to 10 VDC. The amplifier should have a frequency response of about 1Khz, but signals are generally static, but under some applications they can be dynamic. Our main intent make our signal conditioner small enough to embedd in some of our product. Our current signal conditioner is an external device, and it won't give us a 10 Volt output, since it uses 10 volts to supply the amplifers. Thank You For Your Help.

 

[nbucska]

Drive the sensor with +9V and GND. This reduces the sensitivity so increase the INA-s gain. Use a separate accurate 9V regulator. (or calibrate the INA-s gain )

Use a switching regulator for positive 12-14 V for the INA and the 7662. The 7662 must supply only the INA, voltage is not critical.
<nbucska@pcperipherals.com>

 

[morizabal]

I spoke with my superiors and they told for know it won't be necessary to have a 10 Volt Output. What I have done is to use a REF102 10 Volt precision regulator with a PNP transistor to power the INA and 7662. I can get a 10 or 5 Volt excitation, and an output up to 9 VDC. Thank You again for your help.