OPA2251: Re: Circuit simulation in TINA.

Hi Sushant,

It would be very helpful if you could post your TINA .TSC file so we can test the circuit without having to rebuild the schematic.

Examining the schematic you have shown it is seen that you wish to use a single, +3.3 V supply. That is fine, but there is a problem if you intend the apply an ac signal to the U3 non-inverting input without adding a dc offset to the ac signal. Without the dc offset the U3 input will be driven below 0 V and the output which can't swing below 0 V will clip somewhat above 0 V. You should be able to sum in a voltage derived from the +3.3 V to move the input off of 0 V.

The OPA2252 is a precision micro-power op amp and should be suitable for the application providing the input common-mode voltage range VCM isn't exceeded by the ac input signal. The OPA2251 top-end VCM is (V+) - 0.8 V, or +2.5 V with a +3.3 V supply. Another option would be to use an LMC6462 CMOS rail-to-rail input/output op amp. The LMC6462 is also a precision micro-power op amp.

Check to make sure that the .LIB extension which is intended for PSpice use is recognized and usable with the simulator that you mention. Do note that TI now provides TI Pspice, which does support .LIB files. See the following:

https://www.ti.com/tool/PSPICE-FOR-TI

https://e2e.ti.com/support/tools/sim-hw-system-design/f/234/p/926889/3424913#3424913?jktype=e2e

https://community.cadence.com/cadence_technology_forums/f/pcb-design/20494/how-to-create-a-new-pspice-model-library

Regards, Thomas

Precision Amplifiers Applications Engineering

Hello Thomas,

 I was actually providing a DC offset when simulating using another spice simulator.

As the required current is an AC signal, I suppose if I provide an input voltage with AC sine wave, the output will also be an AC current.

I have simulated by providing a sine wave voltage signal at VG1 and I am able to get the AC current at the output.

Can you suggest me any IC/ design which can generate a sine wave signal at a frequency of say about 30-35Hz?

Hi Sushant,

I would give this a try:

sushant_opa2251_wien.TSC

Kai

Hello Kai, Thomas,

 Thankyou for your reply.

Please help me with the OPAMP selection for the same.

As discussed, the input voltage will be a sine wave 1Vp-p, 30Hz frequency.

The output needs to be 6nA AC.

I'll be using a bipolar supply. So there's no need of any offset.

Which OP-AMP would be best suitable for the application? Please suggest me the part number of the same.

Also , for the wein bridge oscillator that Kai has suggested, by checking the properties of OPA251 it can be used for the oscillator. If at all Ti can suggest any other alternate that is best suitable for the application, that will be appreciated.

Also, I need to use an OP-AMP as a buffer.

The input to the OP-AMP will be between 1uV to 100uV, frequency ranging from 0.5Hz to 100Hz.

The device it will be used is an EEG signal measurement device.

Can you suggest me a suitable part number for the same.

To summarize, I would need the best matched part numbers of the below:

1. OP-AMP used in the current source ckt (ckt attached)

2. OP-AMP used in the wein bridge oscillator (ckt suggested by  Kai)

3. OP-AMP used as a buffer.

A brief about the product:

1. Product will be battery powered, so the current consumption of the OP-AMPS needs to be less, also the Iq should be very less.

    The noise density, offset voltage, bias current also needs to be very less.Slew rate also needs to be considerably higher.

Awaiting for the part numbers of the OP-AMP

Regards,

Sushant Rajbhar

EI_current source 6nA.TSC

Hello Sushant,

Low operating current op amps usually have higher noise and lower slew rate than can be obtained with a higher current op amps. If you need higher slew rate, lower noise than can be obtained with the OPA2251 or LMC6462, then a somewhat higher current op amp will be required. I do think whatever op amp is eventually decided upon can be used for all the functions; the V-to-I, sine oscillator and buffer.

Your mention you will be using a bipolar supply. Can you let us know what the +/-V voltages you plan to use? Additionally, do you have some idea what the maximum quiescent supply current can be? Once we have a better idea of the power requirements then that helps us to better provide you with amplifier suggestions.

Regards, Thomas

Precision Amplifiers Applications Engineering

Hello  Thomas,

The supply to be used here is +-3.3V .

The Iq needs to be low. I do not have a figure currently.

I understand there would be a trade off between the power and the noise.

Please suggest the best possible solution which can be used for EEG applications.

I believe as the signals in the system are very low, low noise should be the primary criteria. As the frequency is in the range of some Hz, the slew rate can be considerably lower.

Also it would be okay if the power for the opamps is marginally greater.

Best Regards,

Sushant