Tool/software:
Hello,
I plan to bias the PGA855 as follows:
VS+,VS-= +-18V
LVDD=5V
LVSS=GND
Can the amplifier of the output stage (A3) be damaged if the (Vin+ minus Vin-)*G is greater than 5V or lower than 0V?
I like the idea of using the output stage as a kind of limiting amplifier to protect the ADC stage afterward. Hopefully without damaging the output stage due to overloading of the input stage of the PGA855.
Thanks
Sariel
HI Sariel,
The PGA855 LVDD+/LVSS- output stage supplies are decoupled from the input stage supplies (VS+/VS-), hence, the output amplifier uses a separate power supply that is independent of the input-stage power supply. When driving an ADC, you can connect the LVDD+ and LVSS- to the ADC +5V and GND power supplies. This configuration protects the ADC inputs from damage due to inadvertent overvoltage conditions.
You can certainly power the PGA855 with VS+,VS-= +-18V input stage supplies, and LVDD+=5V, LVSS-=GND supplies with VOCM set to +2.5V. The PGA855 input stage can be overdriven during a temporary fault without damaging the ADC or PGA855 device. When overdriving the PGA855, the output swing of the PGA855 will be limited by the output stage supplies preventing damage to the ADC. The output stage of the PGA855 will not be damaged when overdriving the PGA855 inputs during a temporary fault as long as the Absolute Maximum Ratings are not exceeded.
The inputs of PGA855 are protected for voltages up to ±40 V beyond either supply during fault conditions. If the input is overloaded, the protection circuitry limits the input current to a value of approximately 4.8 mA preventing PGA855 input damage. Figure 8-2 of the PGA855 data sheet shows the input protection functionality during an overvoltage condition condition.
When the input overvoltage protection engages, the current flows through the internal input protection diodes into the power supplies. In applications where the power supplies are unable to sink current, place TVS or Zener diode clamps (ZD1 and ZD2) on the VS+ and VS- power supplies to provide a current pathway to ground:
Table 7.1 of the PGA855 data sheet shows the PGA855 Absolute Maximum Ratings. This table lists the maximum voltages and currents the device can withstand during a temporary fault condition. Input voltages or input currents beyond the Absolute Maximum Ratings can cause permanent damage. If used outside the Recommended Operating Conditions but within the Absolute Maximum Ratings, the device may not be fully functional, and this may affect device reliability, functionality, performance, and shorten the device lifetime.
In summary, the output stage of the PGA855 will not be damaged when overdriving the PGA855 inputs during an inadvertent, temporary fault as long as the Absolute Maximum Ratings are not exceeded.
Thank you and Best Regards,
Luis
HI Sariel,
Assuming the following case when VS+=18V and VS-=-18V, LVDD+=+5V, LVSS=GND, VOCM = +2.5V, setting the PGA855 on G=16 V/V and overdriving the inputs with VIN+ =+1V, and VIN-=GND.
In this case you are overdriving the PGA855 a differential voltage VDIFF= (IN+) - (IN-) =+1V, while applying an input common-mode voltage VCM= [(IN+) + (IN-)] / 2 =+0.5V.
In this case, the output stage is limited by the output stage supplies, and will saturate approximately around OUT+ = (LVDD+) -0.2, or OUT+ = +4.8V and OUT- = (LVSS-) + 0.2V, or OUT- = +0.2V depending on output load conditions. See output voltage swing specifications on the PGA855 data sheet data sheet.
Overdriving the inputs on these conditions you have mentioned above will saturate the output stage, placing the PGA855 well outside its linear region, but will not cause damage to the PGA855.
Thank you and Regards,
Luis
