# Dual Supply or single supply for analog SPDT switch

#### anand3001

Joined Sep 25, 2009
8
Hello all,
I am designing a signal conditioning circuit for a load cell.
I need to offer programmable gain for the input stage instrumentation amplifier. Its a standard instrumentation amplifier with the gain equation given by G = 1 + (50000/Rg). The input to the instrumentation amp is a bipolar differential +/- 10 mV signal. The gains are in the region of 100 to 400. So the voltage at the output of the instrumentation amplifier is in the range of 0 to +/- 1V or +/-4V. But in reality the signal will be unipolar in nature due to the fact that the dead weight of the weighing pan will always place the signal in the positive quadrant.
My question is Can I use a single supply analog switch for the gain selection or do I need to use a dual supply one? Any help would be highly appreciated.
I am attaching an image file showing the proposed scheme for the gain selection for the instrumentation amplifier.
I also have downstream amplifiers that boost the signal to about 7.5 V. Would the mux/switch used for the gain selection for the downstream amp have to be dual supply or single supply and what voltage?

Thanks
regards
Anand

#### OBW0549

Joined Mar 2, 2015
3,566
According to the data sheet, the ADG1409 can be operated from either a single supply or dual supplies.

You've got a more serious issue with this circuit, though: the ON resistance of your analog switch, which can vary from unit to unit and change significantly with temperature, is in series with whichever gain setting resistor is currently selected. This will result in a gain error, especially at the highest gain where your gain setting resistor is only 125Ω or so.

#### ci139

Joined Jul 11, 2016
1,696
Last edited:

#### soyez

Joined Aug 17, 2020
51
Hello all,
I am designing a signal conditioning circuit for a load cell.
I need to offer programmable gain for the input stage instrumentation amplifier. Its a standard instrumentation amplifier with the gain equation given by G = 1 + (50000/Rg). The input to the instrumentation amp is a bipolar differential +/- 10 mV signal. The gains are in the region of 100 to 400. So the voltage at the output of the instrumentation amplifier is in the range of 0 to +/- 1V or +/-4V. But in reality the signal will be unipolar in nature due to the fact that the dead weight of the weighing pan will always place the signal in the positive quadrant.
My question is Can I use a single supply analog switch for the gain selection or do I need to use a dual supply one? Any help would be highly appreciated.
I am attaching an image file showing the proposed scheme for the gain selection for the instrumentation amplifier.
I also have downstream amplifiers that boost the signal to about 7.5 V. Would the mux/switch used for the gain selection for the downstream amp have to be dual supply or single supply and what voltage?

View attachment 215392

Thanks
regards
Anand
it should be dual supply and voltage of 50V will go better

#### anand3001

Joined Sep 25, 2009
8
it should be dual supply and voltage of 50V will go better
I understand the dual supply part, but I don't get the voltage level. How did you get 50V? Could you explain please?

#### anand3001

Joined Sep 25, 2009
8
According to the data sheet, the ADG1409 can be operated from either a single supply or dual supplies.

You've got a more serious issue with this circuit, though: the ON resistance of your analog switch, which can vary from unit to unit and change significantly with temperature, is in series with whichever gain setting resistor is currently selected. This will result in a gain error, especially at the highest gain where your gain setting resistor is only 125Ω or so.
Yes sir you are right about that. With the instrumentation amplifier I want to place the signal within a general window. The rest I intend to adjust with a calibration circuit. There are other analog switches with RDs on as low as 0.5 Ohm. But they are more expensive and will shoot up the BOM. So its a touch choice. I selected a dual supply switch in the ADG 1409 but as an after thought wondered if a single supply switch would work... In that case there are low RDs ON resistance switches that are cheaper. Also I was considering the fact that the final signal will be uni polar after the weighing pan is in place... I thought I might just have a chance to work with a single supply switch. Would be interested to know your thoughts. Thanks for the reply.

#### OBW0549

Joined Mar 2, 2015
3,566
With the instrumentation amplifier I want to place the signal within a general window. The rest I intend to adjust with a calibration circuit.
Watch out: since the analog switch ON resistance is temperature-sensitive, that could complicate calibration.

I selected a dual supply switch in the ADG 1409 but as an after thought wondered if a single supply switch would work... In that case there are low RDs ON resistance switches that are cheaper. Also I was considering the fact that the final signal will be uni polar after the weighing pan is in place... I thought I might just have a chance to work with a single supply switch. Would be interested to know your thoughts.
I don't see why a single-supply switch wouldn't work, provided its specs all line up with what you need.

#### sghioto

Joined Dec 31, 2017
2,394
Its a standard instrumentation amplifier
INA121? If so are you planning on a single or dual supply?
SG

#### anand3001

Joined Sep 25, 2009
8
@sghioto : That's the question I asked... Should it be dual or single supply?

#### eetech00

Joined Jun 8, 2013
2,569
@sghioto : That's the question I asked... Should it be dual or single supply?
If the signal being processed by the INA is bipolar, then the switch should also have a a bipolar supply or it wont pass a signal that goes negative.