INA121: Basic Application Questions

Hello Joseph,

Thank you for your patience. I have answered the following questions .

1. Can I use my Adruino's 5V tap to power the INA121 or do I have to use +V/-V power supply?

- It is possible to have a supply of +5V to power the INA121. Bear in mind, that the output voltage will be limited depending on the value of Vcm. It is recommended to use the input common mode range versus output range calculator. I have attached the link to the calculator below.

2. From what I have read since my pH sensor outputs a +/- mV, I can set Vref to be added to the sensor's output to offset it output to a positive voltage range as required for my Arduino's ADC ports? Am I correct?

- That is correct. An example of that can be seen in the link below.



3.Can I use my Adruino's 5V tap to supply a voltage divider using 1% Resistors to provide Vref to INA121 or must I use only an additional OA's output to supply Vref?

- It is highly recommended to use a op amp's buffered output to supply Vref. This will minimize loading effects of the difference amplifier stage which would effect the transfer function and CMRR.

4. Per the spec sheet's page 9 Figure 3 discussion I need to provide a input bias current path. Should I use Fig 3's Ceramic Transducer example for my pH sensor set up or the Thermocouple example?

- A pH probe behaves as a very weak battery or cell with pH of the liquid determining its output voltage. The probe itself serves as the source for input bias current of the INA121. The purpose of using an amplifier such as the INA121 is to minimize the discharging of the pH cell. Adding a resistor will surely load the probe heavily and dramatically change its output voltage. There is an example of a pH sensor circuit utilizing the INA116 in the link below.



5. Should I include the spec sheets page 11 Figure 9 addition OPA277 and the Capacitor & Resistor? If so I am confused does the frequency = 1/2PI*R*C indicate that signal noise is cut off Below or Above this frequency?

-If you want to AC couple your INA121 an integrator can be added as shown in page 11 figure 9 of the the datasheet. This creates a high pass filter response where the f-3dB = 1/(2pi*R1*C1)



Best regards,



Errol

Thank you! Very helpful.