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about charge amplifier

pan5313 pan5313
Intellectual 360 points
Other Parts Discussed in Thread: OPA333, OPA337, OPA348

I'd like to design a charge amplifier, and our Electronic components stock has opa333.

Now I am not sure whether opa333 is suitable for charge amplifier?

In the datasheet of opa333 I can not find the input inpedence of opa333?

I find one paper about technical article (zhct096.pdf)on the TI technical article, it uses opa337. what is the difference between opa333 and opa337?

I have a question in my circuit simulation.Why the simulation results of following two circuit are different?

why the gain of differential input is bigger than the single-ended input?

differential input :

single-ended input :

 

  • 0
    TI__Guru 64290 points

    Pan,

    OPA333 input capacitance is given in the PDS table while input resistance can be calculated from the graph-see below:

    Having said that, OPA333 is a Rail-to Rail I/O zero drift precision chopper amplifier that may not be well suited for your application while OPA337 is a CMOS Rail-to-Rail output linear op amp more in line with what you need.

    As far as the gain difference between two circuits is concern, the top circuit has the AC gain controlled by the ratio of the impedance of the parallel combination of C2 and C3 with respect to C1, G=-Zc2||Zc3)/Zc1, while the bottom circuit is a simple inverting amplifier with gain equal to G=-Zc2/Zc1 - see below.  

  • 0 in reply to Marek Lis
    Intellectual 360 points

    Marek Lis 

    thanks for your reply.

    I am sorry I still do not well know about why opa333 is not suitable for charge amplifier.  As you said OPA333 is a Rail-to Rail I/O zero drift precision chopper amplifie, so all the Rail-to Rail I/O zero drift precision chopper amplifie can not be used for charge amplifier? The input resistance of opa333 is near about 2.5E+11 ohm . Do you mean that linearity of   opa333 is not well? Do opa333 is FET input?

    what do I still need to care more about charge amplifier?

    the gain of charge amplifier is depended on C2 and C3, so the Capacitance accuracy of c2 and c3 is very import.

    what is the Capacitor material I should select?

    best regards

    panlinjie

  • 0 in reply to pan5313 pan5313
    TI__Guru 64290 points

    Panlinjie,

    Since your application circuit has its input capacitively coupled, it cares mostly about its AC gain; thus using a chopper amplifier like OPA333 with its precision DC performance is a mismatch with your AC type application - you could still use it but you will pay for the performance you don't need. All our chopper amplifiers are designed on CMOS process.

    As far as the AC gain goes, if its accuracy is important, you should use COG ceramic temperature compensating capacitors, suitable for resonant circuits where stable capacitance and high Q are necessary; they are made of non ferro-electric materials yielding superior stability and low volumetric efficiency.

  • 0 in reply to Marek Lis
    Intellectual 360 points

    Marek Lis,

    thanks for your reply.

    my application circuit is not capacitvely coupled, VG1 and c1 are used to simulate the piezoelectric sensor, actually the amplifier input is charge signal.

    As you said All your chopper amplifiers are designed on CMOS process, does it mean that All CMOS process chopper amplifier are FET input?  The input resistance of opa333 is near about 2.5E+11 ohm, while opa337 is about 1E+13 ohm, what is the reason causes the difference of input resistance between opa333 and opa337?

    best regards

    panlinjie

     

  • 0 in reply to pan5313 pan5313
    TI__Guru 64290 points

    Panlinjie,

    All CMOS amplifiers, including choppers, use MOSFET input transistors (not JFET); in case of regular (NOT zero-drift) CMOS op amps like OPA337, the input impedance is dominated by the reverse-biased ESD protection diodes where the net difference between the upper and lower ESD leakage currents (IB) is typically around 1pA; IB may change across the input common-mode voltage, Vcm, by ~0.5pA (see below); therefore,

    Rin=change_Vcm/change_IB=5V/0.5pA=1E+13

    When it comes to chopper amplifiers like OPA333, the input current (IB) is dominated by the current glitches coming from the input switches (not by ESD diode leakage) and depending on the size of the switches and chopping frequency, IB magnitude is much higher than in case of regular op amp - IB is an integrated (averaged out) current spikes that vary significantly with the change in Vcm - see graph below:

    Therefore, Rin=change_Vcm/change_IB = 5V/20pA=2.5E+11. 

    Forty times higher change in IB (20pA vs 0.5pA) means 40 times lower input impedance in case of OPA333.

  • 0 in reply to Marek Lis
    Intellectual 360 points

    Marek Lis

    the operate temperature of opa337 is -40 to 85°, my active operate temperature may be up to 100°.

    so I would like to know whether there are some other amplifiers which are suitable for charge amplifier, meanwhile its operate temperature is -40 to 125. From TI'web I find that most of opa348's characteristics are ok, but it seams that it open gain is 94dB（opa337is 120dB）, will it affect the  gain accuracy?

    Can you recommend a operational amplifier of Good performance which is more suitable for charge amplifier?

    best regards

    panlinjie


  • 0 in reply to pan5313 pan5313
    TI__Guru 64290 points

    Panlinjie,

    OPA337 is specified from -40C to 85C BUT operates from -55C to 125C - see below.  The difference between specified and operates lies in the fact that in the latter case the part may not meet PDS max/min specs but it works fine as an op amp.

    However, OPA337 typical performance has been characterized from -55C to 125C - see PDS curves below.

    The typical DC open-loop gain (AOL) of OPA348 is 104dB (min 94dB) BUT what is important in your application in terms of gain accuracy is not DC but AC AOL at the operating frequency - at 5kHz, AOL of OPA337 is around 60dB while AOL of OPA348 is 46dB.  Since your application has a close-loop gain of around 1, the loop gain (a difference between the open and close loop gains) is greater in the case of OPA337 resulting in its higher gain accuracy.