# Amplifier circuit for Bently Nevada velomitor 330525

I'm not sure how to design a ampliefier circuit for the signal.

The datasheet is says that it is a piezo electric velomitor, but what is throwing me off is that it has power requirements of DC voltage -22 to -30 and bias current of 2,5 to 6 mA. The output bias is -12 +-3 VDC.

The output is 4mV / mm/s and I would like to amplify this by 100, so it would be 400 mV / mm/s. I cannot figure out how to build the circuit because it apparently needs negative volts and it is only a two wire velomitor.

Could someone point me in the right directions? I have dabbled with basic electronics but nothing really with negative volts and piezos.

Datasheet

• Have you tried searching for this: Velomitor Sensors User Guide (document 100076) - as per the recommendations in the data sheet you linked? Jan 15, 2021 at 15:31

The user guide referenced in the document you linked explains it fairly clearly.

Traditional velocity sensors consist of either a moving wire coil surrounding a fixed magnet or a fixed wire coil surrounding a moving magnet. The Velomitor® Sensor is more accurate than traditional velocity sensors. Because the Velomitor® Sensor contains no moving parts, it is also more durable and less sensitive to transverse motion than traditional seismic transducers. Its piezoelectric sensing element and solid-state circuitry let the Velomitor® Sensor withstand years of continuous use.

The Velomitor® Sensor is a two-wire device that requires an external power supply. The power supply must provide a DC voltage of 22 to 30 Volts and a current of 10 mA. A constant current diode must be used to limit the current to the sensor to 2.5 to 6 mA. Figure 1-1 shows a simple block diagram of the Velomitor® Sensor system. [Emphasis mine.]

Table 3.1 gives the required diode part number:

3 mA (Motorola P/N 1N5309) current diode Bently Nevada P/N 00643485.

Wiring seems to be as simple as this.

simulate this circuit – Schematic created using CircuitLab

Figure 2. General gist of your amplifier.

The datasheet mentions a DC bias on the output so you're going to need to remove that before amplification. C1 does that. You'll need to pick a value that will allow the lowest frequencies of interest through without too much attenuation.

R3 and R4 bias the op-amp to half supply so that your signal can swing symmetrically about that point. You don't say what this is feeding into so I have no idea if this will work in your application.

I had never heard the term before. Thanks for the education.

• Thank you that was extremely helpful. So I can supply it with +24 VDC with the current diode even if it says that it wants -22 to -30 VDC on the datasheet? I'm probably reading it on arduino so I'll modify it to 5V level. Do I need to be conserned about the QA1 input impedance. If so is there anything that you suggest that I would use? Jan 15, 2021 at 18:34
• That's "OA" as in operational-amplifier or op-amp. For most practical purposes the op-amp has infinite input impedance. You'll need a 5 V op-amp with rail-to-rail output in that case to prevent damage to your microcontroller ADC input. I'm not sure what you hope to achieve with this. I don't know that you'll be able to sample at a high enough frequency or do anything useful with the Arduino's limited memory. Jan 15, 2021 at 18:38