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# PGA308: How to adjust the common mode output voltage

Part Number: PGA308
Other Parts Discussed in Thread: AMC3301,

Hello,　team

Here is my application:

We use a TI AMC3301 to sense a highside bi-direction current,  AMC3301’s differential ouput is

Va= 1.44V±50mV  ( common mode output voltage= 1.4V while current=0A)

we want use a tI PGA308 to convert it to a Vcc(5Vtyp) based ratiometric output

Vo= (Vcc/5)(2.5V±40Va

( means Vo=2.5V±2000mV  at Vcc=5V,  common mode output voltage 1.44V -> 2.5V   and  signal span  ±50mV -> ±2000mV by PGA308)

I want to set Font-end amp as Gain=8,  Output Amp as Gain=6  and Fine Gain adjust = 0.8333,   to set total signal  Gain=40,

But I confused of how to arrange the  common mode output voltage Va=1.44V  to  Vo= 2.5V

Thanks  a lot

• To PGA308 question,

The PGA308 is really designed for a bridge sensor input.  A bridge has a common mode signal equalt to approximately 1/2Vexc with a small differential signal.  Although your application is somewhat different than the typical case, I think the PGA308 may work for this.  I can confirm usign the EVM, but I want to check with you on a few items before I do this test.  See the schematic below.  The important requirement that allows this circuit to work is that the AMC output has a fixed common mode of 1.44V.  This common mode is canceled by using the fixed voltage reference on the PGA inverting input.  As long as the AMC common mode is constant and we can replicate it in the inverting input using a reference circuit, the PGA308 should work.  Let me know if the 1.44V is constant, and if so, I will confirm this using the EVM.

Best regards, Art

Let me describe it again:

The AMC3301 has a diff outut as OUTP, OUTN,   with constant common mode voltage(=1.44V)

at my app,  the AMC　output(= Input of PGA308) :  as a V(diff)=±50mV

Case1:     Current  = + direction:     OUTP= 1.44V+25mV,    OUTN=1.44V-25mV,

Case2: 　Current =  - direction:    OUTP= 1.44V-25mV,    OUTN=1.44V+25mV,

( the input  is absolute voltage )

The Output (VPGAout = output of PgA308)  I want is full Vcc based ratiometric value:

When  Vcc=Vref=5V

Case1:  VPGAout   =   2.5V+2000mV

Case2:  VPGAout   =    2.5V-2000mV

When  Vcc=Vref=5.1V

Case1:  VPGAout  =  (5.1/5) * ( 2.5V+2000mV)   =     2.55V+ 2040mV

Case2: VPGAout  =  (5.1/5) * ( 2.5V-2000mV)   =     2.55V- 2040mV

Thanks a lot.

• To PGA question,

Below is a step-by-step procedure to accomplish your goal.  Before trying the external AMC3301, please confirm that the hardware and software works correctly by doing the standard calibration with the sensor emulator.  The power point steps through the standard sensor emulator calibration.  Next make the jumper changes and connections as directed by the document.  Also make the software changes indicated in the document.  Finally step through the calibration.  I used your input signal levels and achieved a very accurate calibration result.  After calibrating, you can apply the signals for post calibration verification of accuracy.

pga309-cal-input-from-AMC3301.pdf

I hope this helps resolve your PGA308 system issues.

Best regards,

Art

• Thank you very much for your quick response and the reply  so detail.

In fact , this is my first time to use this kind of  part,  a littel complex  as PGA308,

I'll purchase a EVM kit to evaluate it based on your reply.  I think it will work well.