TLV3404: Piezo element // 2 stage op-amp design

Sketch below

Jeroen

from my ultrasound days, we developed a product called the TDC1000.  since this product was used for multiple applications, it contains the ability to excite the piezo sensor and receive a signal back.  It sounds like you are using the piezo in microphone mode only, and will not be exciting it.  If that is the case, only the receive portion of the product will be relevant.  the tlv3404 is a quad comparator, so that alone will not satisfy your requirements.  Please clarify for those monitoring this post if their is any excitation being applied to the piezo sensor and what supply voltage is available.

Chuck

Hi Kai, Paul, Chuck,

Thanks a lot for your feedback - below some additional info:

• Piezo sensor is a long element (PVDF) which can be placed on the street
• Piezo sensor is 2 meters plus additional coaxial cable to hook it up to my test system
• In my test I use the sensor for bikes only
• When a bike crosses the piezo sensor the output is approx. between 20 - 50 mV (depending on weight, speed etc.).
• When no bike crosses the piezo – output is approx 0 mV
• Waveform example below (this is a car!) – see also http://www.te.com/content/dam/te-com/documents/sensors/global/TE_SensorSolutions_RoadTrax-BL-Piezoelectric-Axel-Sensor_SS-TS-TE600.pdf

@Kai – comparator vs OPA. As said I am not an opamp expert but for this scenario could I not use the TLV3404 as both comparator and OPA? (afaik they are optimized differently (??))

• Power consumption for opamp design– max in range of uA when “monitoring” the piezo sensor
• My processor sleeps at approx. 1uA. System going in mA range for power consumption is a no go when powered by batteries
• Battery powered – voltage 3.6V
• Comparator threshold - 2V (but we could lower it when a smaller gain is used). Piezo input above 20mV should trigger comparator

@Paul – maybe a gain of 20 – 40 makes more sense (and make comparator threshold lower)

Jeroen

Hi Kai, Paul,

I am impressed with the design suggestions // seems like a good solution. Have some questions about it:

1. Total power consumption is around 50(uA).
   1. This is ok but more than I have in mind (would like to go below 10 uA)
   2. Therefore I am wondering if it would be possible (or not) to use other nano-power op-amps like the TLV8811, TLV8802, TLV340, TLV 8541 etc. (in combination with a comparator like the TLV3012 or the  TLV340x)

2. I always see op-amp gains with different values (small (kOhm) or large (MOhm)).
   1. Do you know what the design criteria is in such a low power op-amp design or what it is in general?

@Kai - would it be possible to send me the TINA file?

Let's see if we can get this design working (will keep you up 2 date).

Thanks,

Jeroen

Hi Jeroen,

first, when I looked at the scheme Paul was proposing, I thought to me that one should be a able to use an OPAmp offering a much lower supply currrent. But I was wrong! If you take such an ultra low power OPAmp you will not get the unity gain bandwidth and slewing rate necessary to handle the piezo signals. A unity gain bandwidth of at least 1MHz seems to be mandatory. So, the only OPAmp which was as fast as Paul's TLV9002 and was drawing a bit less supply current at the same time was the OPA348. So, I totally agree with Paul.

You see that I have used rather high resistance values to be able to save supply current. But an additional increase would not help much, because the most supply current is drawn by the idle current of OPA348. The consequence of using high feedback resistances is that you must do something to improve the stability again. That's why C1 and C2 are needed. In a design with much lower feedback resistances these caps could be omitted.

jeroen_piezo.TSC

Kai

Hi Kai, Paul,

Before I create a test PCB, I have build a test setup on a breadboard.

To test the first stage op-amp and to check the piezo signal I used an LM6134 (had nothing available in DIP package)

Two things which I noticed on the scoop (see below)

1> The piezo signal seems to be stronger when pressed and more symmetrical, so in the end I might want to use a smaller gain

2> Feeding the piezo input to the LM6134 (PIN3) with gain of 10 (100k / 10k) and VCC 3.6(V) -->  I have an offset of around 480 (mV)

So maybe you have some ideas where the 480(mV) comes from and what is the best way to get ride of this?

Rgds, Jeroen (not John :-))

:

Can you measure the output signal of the piezo directly (ie before the amp chain)? How much DC offset is there present at the input of the op-amp? The LM6134 has max offset of 2mV so I would only expect about 20mV of DC offset due to the op-amp itself.

-Paul

Hi Paul, Kai,

The offset when only measuring the piezo is 0(mV) - see picture below. So the offset somehow (imho) comes from the LM6134.

Kai, you mentioned that the used op-amp generates too much input bias current (datasheet says 'typical 110(nA)' - and over 1M resistor this is 110(mV)) which indeed is what I see (in the above 480(mV) a larger resistor was used).

Do you have a suggestion which op-amp I could use to test the circuit as a whole (for breadboard testing DIP package only / no SMD pls :-))

Jeroen

Hi Kai,

Not using the OPA348 for the simple reason that it does not come in a DIP package. I am only used to work with breadboard before creating a PCB protoboard.

(maybe silly question but are there other ways to test non-DIP packages?)

The TLC272 comes in a PDIP package so that is one I can use to test.

Thanks

Jeroen

Received the adapters and got the circuit working with the OPA348 and the TLV3012. The detection of the piezo signal goes very well.Have connected the output directly to a micro processor and signal is received.

However there is one issue which I have not found a solution for. After measuring on the oscilloscoop I found out that when the output (comparator) goes from LOW to HIGH the first 50 usecs the signal bounces a couple of times.Not sure what the best measurements are to eliminate this?