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Low-Offset, Low-Noise, Wide-Bandwidth Chopper

Most of the well-known semiconductor companies have chopper amplifiers in their portfolios. The chopper is a device that modulates the input signal with a carrier with a chop frequency (fchop ) of perhaps a few hundred kHz. The input signal is probably DC or low-frequency AC. The chopped signal is amplified (probably a lot) with an AC amplifier. The resulting signal is demodulated. This yields amplification of the input signal without the pesky DC artifacts caused by DC offset (or DC offset drift) errors that plague typical high-gain, general-purpose op-amps.

Today's choppers show much improvement over devices from just a few years ago. But there is always room for better specs. A recent IEEE/ISSCC paper (registration required) describes a way to get not only low offset voltage (3μV) but also a very wide input common mode voltage range (CMVR). They also solved another annoying problem:

A significant drawback of such [chopper] amplifiers is a transfer-function notch around [fchop ]. This is because their input choppers demodulate signals near fchop to DC, where they are blocked by [coupling] capacitors. This problem is exacerbated by the use of a ripple-reduction loop (RRL) to suppress chopper ripple, which also creates a notch at fchop , and, moreover, can take up to 1ms to settle. The net result is an amplifier with a transfer-function notch and a step response that is accompanied by a slowly decaying burst of chopper ripple.

To fix these problems, the authors propose “a multi-path capacitively-coupled chopper-stabilized operational amplifier (MCCOPA).” The proposed circuitry should produce a 20V CMVR and a common mode rejection ratio (CMRR) of 148dB.

The architecture consists of high- and low-frequency paths. The high frequency path (HFP) is a simple AC-coupled amplifier (gain of 14μS) feeding the output stage (a Miller integrator). Its inputs are capacitively coupled, of course, so DC offset doesn't matter. The low-frequency path (LFP) consists of a chopper cell, AC coupling, an amplifier stage, another chopper cell acting as the demodulator, and some additional gain stages. The signal from this path is combined with the AC-coupled signal in that same Miller integrator stage.

There is an additional side chain around the DC amplifier stage that feeds back a small portion of the signal via another chopper cell. This is the RRL. It reduces the residual amount of chopper-induced ripple in the output of the amplifier.

Here is the block diagram showing these three signal paths (high, low, and RRL).

And here's a detailed view of how that is squeezed on to the chip.

The authors also explain how their design works to suppress 1/f noise. The crossover frequency (the breakpoint between the HFP and the LFP) is above the corner of the 1/f noise. The 1/f noise in the LFP is suppressed by the HFP, and the noise in the HFP is suppressed by the LFP. Thermal noise caused by the large value resistors is similarly suppressed.

The specs at a glance:

  • The CMVR is -0.6V to +20V (limited by ESD diodes and input pads).
  • The input offset voltage (based on 14 samples) is less than 3μV.
  • The input bias current is less than 107pA.
  • The input offset current is less then 95pA.
  • The PSRR is more than 120dB.
  • The CMRR is more than 148dB.
  • The fchop value (input-referred residual ripple, AV =100V/V) is 125nV (mean amplitude) and 280nV (peak amplitude).
  • The input-referred noise density is 55nV/√Hz and is flat down to ~1Hz.
  • The power supply current draw is 8μA @ 5V.
  • This device is fabricated on a 0.7μm CMOS process, with a chip area of 1.35mm2 .

Here's a better view of the noise and the frequency response. On the left, we have output noise spectrum density at a gain of 95V/V. On the right, we have frequency response with and without the multipath architecture described above. The gain is 22V/V; CL is 40pF.

And this look at the step response under a variety of conditions should provide everything you need to know about its slew rate.

Lastly, here's a comparison with several other recently designed devices that showed up in other IEEE/ISSCC papers.

Have you used ultra-low offset op-amps, chopper or otherwise? What was the design experience like? Did the devices perform as expected? Did they meet your requirements?

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13 comments on “Low-Offset, Low-Noise, Wide-Bandwidth Chopper

  1. Scott Elder
    July 19, 2013

    Hi Brad – The table showing figures of merit (FOM) is a bit misleading–it's not clear why that is even in the paper.  One can't claim a FOM victory (i.e. 100 for this work) based upon bandwidth and power when the minimum gain for stability is greater than the comparison works (AV=20 vs. unity gain stable) and for less capacitance (40pF vs. 100 pf).

    The author also talks about great improvements relative to [1], but again the noise is lower in [1] [which means more supply current] and the stable load capacitance is greater (i.e. 70pF vs. 40pF).  It takes more power to drive a larger capacitive load, more power to do this while stable at unity gain, and more power to achieve lower broadband noise.

    Ref [4], done 7 years ago, if I'm not mistaken, is the OPA333 from TI.  It is NOT a 1.8V max ampliifer, but 5V.  They just promoted it in the paper as 1.8V to emphasis how low one could go. 

    It seems the real difference in this product is the 20V common mode from a 5V supply.  Why not leave it at that?  I start to wonder about the value of a work when the author focuses attention in an area where apples and oranges are compared.

  2. Brad_Albing
    July 19, 2013

    There is a tendency at salesmanship with some of these presentations. I suppose the IC designers have a vested interest in getting some manufacturer to purchase the IP rights to the IC. Justmy 2 cents worth….

  3. Netcrawl
    July 20, 2013

    Its getting more manufacturer's attention, trying to lure them to purchase this IP rights, its common this days, it really happen, IP rights is big serious money. Companies know the potential power of IP rights and the rewards, and the best thing to sell it is a good marketing stuff.  

  4. WireMan
    July 20, 2013

    “…and the best thing to sell it is a good marketing stuff.”  Truer words were never spoken. Having been subjected to many “marketing stuffs” I understand completely. 😉

  5. Davidled
    July 20, 2013

    I recalled that field engineer visits to company and he got some error related to window 7 OS and coaxial while demonstrating the module related to low noise amplifier. In the end, we did not see any performance of low noise amplifier due to the error of testing set-up. It was realized how important sale and field engineer contribute to their product sale, since this date.

  6. Brad_Albing
    July 21, 2013

    @WireMan – Here too. Well, to be fair, I've not only been subjected to lots of marketing stuffs but also done my fair share of presenting marketing stuffs.

  7. TheMeasurementBlues
    July 23, 2013

    “done my fair share of presenting marketing stuffs.”

    Spoken like one from the dark side.

  8. Brad_Albing
    July 23, 2013

    Indeed. And it's easy to see that I make a living putting words together goodly.

  9. Cookie Jar
    July 24, 2013

    What the world needs is not more chopper stabilized op amps, but chopper stabilized INSTRUMENT AMPLIFIERS.  

    My 1981 Intersil data book shows an ICL7605/ICL7606 chopper stabilized instrumentation amplifier (They called it a Commutating Auto Zero (CAZ) just to be different.)  I used them with a load cell and their performance of less than 1uV offset through the extended temperature range was most impressive.  All you needed was ONE low tempco resistor to set the gain.

    The irony is that a good percentage of the chopper stabilized op amps are in fact used in the Instrumentation amplifier configuration.

    When  you use a chopper stabilized OP amp, you are stuck with using a bunch of op amps as well as a bunch of DISCRETE low tempco resistors to implement the instrumentation amplifier configuration of your choice.  This uses a whole lot more real estate on your board,  ups the cost of expensive low tempco resistors as well as often necessitating resistor trimmers.  In addition, thermal gradients in your spread out board layout mess up temperature performance compared to a single integrated device with only one expensive resistor.  

    An integrated chopper stabilized Instrumentation amp on the other hand has most of the resistors within the IC, pre-trimmed and temperature tracking – so much more elegant.

    I fume at the never ending introduction of chopper stabilized op amps, with lousy performance compared to a 30 year old obsolete device.  It makes the expression one step forward, two steps back ring true.

  10. Brad_Albing
    July 29, 2013

    @Cookie jar – We will hope that the IC manufacturers that read these words (and cling to every morsel we dispense) take to heart your suggestion. To me, it does seem quite sensible to make a chopper IA – like you said, eliminate the need to match a bunch of [precision] resistors.

    The key of course is the needs of the marketplace . So, we'll see….

  11. Scott Elder
    July 29, 2013

    @CookieJar – is it possible to use Delta-Sigma ADCs directly now (i.e. 24 bits) rather than the 80's instrumentation amplifiers, or are the voltages and CMRR still what drives using IAmps?

    Scott

  12. SunitaT
    July 29, 2013

    The TLC2654 and TLC2654A are low-noise chopper-stabilized operational amplifiers using the Innovative LinCMOS procedure. Relating this process with chopper-stabilization circuitry makes excellent dc precision conceivable. In addition, circuit techniques are added that give the TLC2654 and TLC2654A greater noise performance

  13. Brad_Albing
    July 30, 2013

    @Sunita – have you used those in a design? How well do they perform?

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