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# PGA309 Design

G’Day

We are in the process of designing a PCB based on the TI reference design - TIPD126_SCH_revA.  The PGA309 will communicate with a 8051 mcu on I2C for access to the eeprom registers.  The Vout will be read by an ADS1110, also with I2C.

Parameters are:

Internal Vref:     2.5V

Internal Vexc:    2.075V

Rbridge:            1000ohm

Vout_z:               0.2V

Vout_fs:              1.224V

V/V:                     1mV/V

Vsa:                     5V

Course Gain:      -8.500mV (calculator)

FE PGA Gain:      128 (calculator)

Zero Dac:             1.140V (calculator)

Gain DAC:            642.570m (calculator)

BE PGA Gain:     6 (calculator)

4-20 mA circuit:

R4 + R5 = (Vout_max - Vout_min) / (Iin_max – Iin_min)

= (1.224 – 0.2) / (0.0002 – 0.00004)

= 1.024 / 0.00016

= 6.4Kohm

R4 = 3.16Kohm

R5 = 3.24Kohm

Iin_min = Vout_min / (R4 + R5)

= 0.2 / 6.4Kohm

= 0.00003125mA

Iref         = 0.00004 - 0.00003125

= 0.00875mA

R2           = 2.5 / 0.00000875

= 285.714Kohm

~ 287Kohm

C10         = 1 / (2 * pi * R4 * 1.41) * 1000

= 35nF

~ 33nF

fc_act    = 1 / (2 * pi * R4 * c10) * 1000

= 1.526KHz

I have some questions regarding the design:

1. In the reference design, TIPD126_SCH_revA, there is no resistor in the Vfb.  I would have expected to see the same 100ohm as R3?  Any reason why?
2. Is D1 (SMBJ5V0CA) really necessary since the Vreg from XTR117 supplies a regulated 5V? D1 is quite a big component for small PCB.
3. Is the fc_act of 1.526KHz sufficient for a low-pass filter on Vout.

Thanks

Cheers

Dirk

• Hello Dirk,

1. An additional 100Ω feedback resistor is not necessary, since a feedback resistance network already exists internal to the device around the output amplifier. We do not use a resistor here on TIPD126 or on the PGA309 evaluation module (EVM). We do use a feedback capacitor - C7 on TIPD126 - to limit the bandwidth of the output amplifier for noise reduction purposes.
2. This circuit was designed to be extremely robust against transient voltages and currents, in order to pass the full suite of IEC certification tests. D1 is one layer of protection circuitry in this system - the idea behind this component is that even if the XTR117 is damaged and its internal regulator puts out a large transient voltage, D1 will shunt that transient away from the PGA309.
3. A cutoff frequency of 1.526 kHz should be sufficient. It's a cutoff very close to the one I used on TIPD126 (1.4 kHz).

Best regards,

Ian Williams
Linear Applications Engineer
Precision Analog - Op Amps

• In reply to Ian Williams:

Hi Ian

Much appreciated....

Thanks

Cheers

Dirk