High voltage diff amplifier

NJU for analog high voltage signal pre-processing in industrial and automotive applications. While these kind of amplifiers do NOT feature galvanic isolation they have a very high impedance voltage divider in front of the actual OpAmp. With this internal resistor circuit the NJU can scale down input signals of some hundred volts to the supply voltage level of 2. The power supply is common to both amplifiers.

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Content:

• A 450°C High Voltage Gain AC Coupled Differential Amplifier
• High Voltage Amplifiers
• Differential High voltage measurement
• Electronic – High Bandwidth, High Voltage differential amplifier problems
• What is the common-mode input voltage of an op-amp?
• High-Voltage Differential Probe
• Probing Small Signals on a High-Voltage Bus
• Difference Amplifiers

WATCH RELATED VIDEO: Analog Devices LT1997 Precision High Voltage Difference Amps

A 450°C High Voltage Gain AC Coupled Differential Amplifier

Home » High-Voltage Differential Probe. A high-voltage differential probe is an indispensable piece of test equipment for anyone who wants to examine high voltage signals on a standard oscilloscope and do so safely. For safety reasons, the ground side of your oscilloscope probe is connected directly to mains earth. This means you can only measure earth-referenced signals, or truly floating signals, such as those encountered in battery-powered circuits, where it is possible to connect one part of the circuit to mains earth via the scope.

But what if we want to measure some signals in a mains-referenced circuit such as an off-line switch-mode power supply? The control circuit is typically referenced to the negative side of the rectified mains. This point is certainly not at earth potential. If you were to connect the ground clip of a standard oscilloscope probe to this point, you would create a short directly to earth via your oscilloscope. This would almost certainly damage your oscilloscope and probably destroy the circuit under test.

Incidentally, when I started my career many years ago in the power electronics field, you would routinely see oscilloscopes with the earth wire cut at the mains plug to avoid just this problem. This was—and remains—an extremely dangerous practice, because it meant that the entire oscilloscope would be floating at mains potential. Merely touching the case of the scope could deliver a fatal shock. Fortunately, there is a safe way to measure high-voltage circuits such as in this example: a differential probe.

It has two high-impedance inputs and a single, ground-referenced output. The output is proportional to the difference in voltage between the positive and negative inputs. Any common-mode signal is rejected. The output would be proportional to the difference between the two. I thought it would be nice to build a differential probe that was reasonably small sized with a rechargeable battery. It has proved to be an interesting exercise in precision analog design, as you will see later.

In principle, the concept of a differential probe is pretty straightforward: a matched pair of input attenuators followed by a classic three op-amp differential instrumentation amplifier, as shown in Figure 1.

You can think of this circuit has having three sections: an attenuator, a buffer stage and a difference amplifier stage. The overall gain of the circuit is given by multiplying the gains of each of these stages as shown in this equation:.

Of course, this means smaller signals—such as the gate drive in our notional power supply—will be just a few tens of millivolts after the attenuator. Clearly, we need the following stage to optionally provide some gain when we are looking at small differential signals. The good news is that the gain of the buffer stage of a classic instrumentation amp such as this can be programmed a single resistor, Rd in Figure 1.

If Rd is open circuit, the gain of this stage will be unity, suitable for input signals in the multi-hundred-volt rage. By switching in a resistor Rd we can add an additional range with a gain of perhaps 10, allowing us to sensibly measure differential signals in the tens of volts. The final stage is the difference amplifier, which converts the differential signal into a single-ended ground-referenced one. So far, so good.

I now had three analog stages to design, plus a power supply. Achieving greater than 60 dB CMMR common mode rejection ratio means matching the attenuators and the resistors in the difference amplifier to at least 1 part in 1, Achieving a large-signal bandwidth greater than 25 MHz would require some pretty special op amps and careful attention to parasitic capacitances and board layout.

I also had to contend with all the usual non-ideal characteristics of op amps such as input offset voltages, input common mode ranges, input bias currents and the like. Sounds like fun, so here I will walk through the design process pretty much as I tackled it.

Series resistors are required here, since the maximum working voltage of a single resistor of the type I chose is limited to V. A quick check shows power dissipation in this resistor string will not be an issue up to 2. This comfortably exceeds our design requirement of V. The value of Rb can now be calculated to be The schematic in Figure 2 shows this pretty odd value is made up of a string of resistors and a trim-pot.

This turns out to be a much cheaper way of buying precision resistors than as individual components, though with a limited range of values. As an added bonus, these resistors are matched to each other within 0. This helps keep our attenuators matched. The astute reader will have already observed that this combination of resistors adds up to R12 is associated with op amp offset nulling, discussed later, while R15 is there purely to keep the circuit symmetrical.

The diode pairs DP1 and DP2 are there to protect the op amp inputs from any overvoltage making its way through the attenuator. They effectively clip the signal to one diode drop above or below the supply rails. This is all well and good for DC signals. We have a safe circuit with precise attenuation and great CMRR. But what about the AC behavior? The buffer op amps will have a small but finite input capacitance, as will the protection diodes. According to the datasheets, this combined capacitance of the op amp and protection diodes will be on the order of 4.

Not much, you might say. But the corner frequency of this low-pass filter will be under 10 kHz. The impedance of this and the parasitic capacitance should be in the same ratio as the resistances in the voltage divider. In our case, the compensation capacitors should be times smaller than 4. Obviously, this is not practical, so we have to tackle this problem from the other direction.

This gives us a compensation capacitance of 2. For proper frequency compensation we now need a total capacitance at the input to the buffer op amps of The trimmer provides a range of adjustment of pF to pF to allow for component tolerances and layout differences. The total capacitance seen at the input terminals is nominally 2. This classic circuit has a few very nice features that come in handy for this application. It has very high input impedance, common mode gain of 1 independent of resistor matching, and differential mode gain programmable via just one resistor as already discussed.

The technical requirements for the input op-amps for U1 and U2 are pretty tough. If I only required a gain of 1 for this stage, I could have simply wired these op-amps as non-inverting buffers. Since I wanted to have the option of a gain of 10, I had to close the feedback loop around each op amp with a resistor R18 in the schematic and Rc in Figure 1. It is a good idea to choose a fairly low value for this resistor, because it will form an RC low-pass filter with the op amp input capacitance—the effect of which will be to increase the gain of the buffer as the frequency rises.

This minimizes the effect of gain peaking in the frequency range of interest, but there is still a potential issue at high frequencies that should be addressed.

It forms another low-pass filter that attenuates the input signal at about the same rate as the low-pass filter in the feedback loop amplifies it. This turned out to be the infinitely repeating value of You will note from the schematic that I have specified a 0. This is a trick well worth keeping in mind for your precision designs. Just as for the input attenuator, I had to provide frequency compensation for the voltage divider formed by Rc and Rd Figure 1.

This is a series connection of pF and 10 nF capacitors. The final difference amplifier is, by comparison, fairly simple to design. The rejection of any remaining common-mode signal relies on well-matched components, so again I took advantage of low-cost matched resistor arrays for R22 and R For this op amp, I could relax the requirements a little compared with U1 and U2.

A lower cost CMOS op amp should suffice here, so one would expect fairly low input offset voltages compared to the FET input op amps used in the previous stage. I did, however, need a pretty good large signal bandwidth. Typical input bias current is Input common mode range is I chose to use a single cell LiPo battery as the power source, since this could be charged from the standard USB power supplies that have become ubiquitous.

An AA-sized 14, cell would fit nicely in the case I had in mind. There are a few ways to do this, but I figured I could not be the first person with the need for dual 5 V supplies from a single LiPo cell. This nice little chip accepts a 2.

It requires only a couple of inductors and three low ESR ceramic capacitors. The chip also has a low-voltage shut-down to protect the LiPo cell from over-discharge. This is a very simple linear charger with two inputs, intended to be used in applications where there is both a USB power source and a DC wall wart. Note that the power switched via SW1A. In either the x10 or x gain position, the battery is connected to the switcher and isolated from the charger.

This means it is not possible to both use and charge the probe. This was a deliberate choice on my part to keep the modes completely separate and encourage me to keep my bench as clear as possible. Testing shows the circuit can achieve 3 hours and 45 minutes of operation on a full charge with the mA-hours LiPo I used. Charging takes a couple of hours on the high-current setting, and about 6 hours on the low-current setting. You could substitute an cell if you reconfigure the mechanical design, which should more than double both the running and charging times.

Figure 3 shows a view of the case. Two 4 mm banana input sockets protrude through one end of the case, and the BNC output connector and USB Mini-B connectors protrude through the other end. The label was made using a laser printer label and transparent sticky plastic film.

The board layout is critical. Trust me, I learned this the hard way.

High Voltage Amplifiers

Kind code of ref document : A1. Effective date : A high-voltage differential sensor includes an attenuator formed of two matched monolithic capacitance divider networks C1,C2. Each divider network is formed of a series connection of monolithically integrated capacitors C1 , which together generate an attenuated differential signal from a high-voltage differential input signal. The attenuated differential signal from the capacitance divider networks is then amplified and fed to a comparator 88 , which generates a first output level when the high-voltage differential input signal is above a selected level, and generates a second output level when the high-voltage differential input signal is below the selected level. By using monolithically integrated capacitors in the divider networks of the attenuator, a simple, compact, low power, high performance high-voltage differential sensor is obtained.

Differential High voltage measurement

The HA51U series is a family of single channel, bipolar, unipolar and unsymmetrical high voltage amplifiers. They feature high speed, high precision and high stability as well as very low ripple and noise. They are designed to drive capacitive and resistive loads. High peak output current facilitates easy driving of capacitive loads. The output stage is fed by internal high voltage sources. The amplifier output is protected against overcurrent, short circuit, overvoltage, overtemperature and high voltage flashover. A safety interlock feature is provided to integrate the unit into a safety circuit. An isolated USB interface is provided to control the amplifier by means of a simple command interface setting of the output voltage, monitoring of output voltage, current, temperature and further operational parameters, configuring the amplifier.

Electronic – High Bandwidth, High Voltage differential amplifier problems

You might consider that common-mode signals are never applied to an op-amp. This is equivalent to applying common-mode signals or signals with little difference in voltage to the op-amp. If the input signals of an op-amp are outside the specified common-mode input voltage range, the gain of the differential amplifier decreases, resulting in a distortion of the output signal. If the input voltage is even higher and exceeds the maximum rated differential input voltage, the device might deteriorate or be permanently damage.

What is the common-mode input voltage of an op-amp?

Electricity is very hard to imagine because we can not see if a voltage is present or if a current is flowing. If we want water to flow out of a pipe we need some water pressure which is achieved with a water pump. In electricity, our flow is the current, water pressure is the voltage and the pump is the battery. This means that the voltage is the cause of the current. Voltage , sometimes also called electromotive force, is what makes electric charges move.

High-Voltage Differential Probe

I inherited a opamp measurement circuit that is supposed to be able to measure differential voltages of up to V with a bandwidth of 1 MHz and a protective impedance between the HV and SELV side. The V on the primary side are converted to 1. It is basically a differential amplifier with 9 megaohm resistors so that no lethal currents can flow. The 1. To prevent troubles with the input capacitance of the opamp or the capacitance of the diodes pF capacitors are used to add a defined capacity again in both paths because of possible common mode problems otherwise.

Probing Small Signals on a High-Voltage Bus

The AD is available for sampling and is now in full production. The AD offers cost-effective isolation and can replace more costly devices in applications that do not require galvanic isolation. The AD is a difference amplifier with a very high input common-mode voltage range. The AD can replace costly isolation amplifiers in applications that do not require galvanic isolation.

Difference Amplifiers

Home » High-Voltage Differential Probe. A high-voltage differential probe is an indispensable piece of test equipment for anyone who wants to examine high voltage signals on a standard oscilloscope and do so safely. For safety reasons, the ground side of your oscilloscope probe is connected directly to mains earth. This means you can only measure earth-referenced signals, or truly floating signals, such as those encountered in battery-powered circuits, where it is possible to connect one part of the circuit to mains earth via the scope. But what if we want to measure some signals in a mains-referenced circuit such as an off-line switch-mode power supply?

Skip to search form Skip to main content You are currently offline. Some features of the site may not work correctly. DOI: Killat Published Computer Science 38th International Convention on Information and Communication Technology, Electronics and Microelectronics MIPRO This paper presents the design of a high-voltage differential amplifier using six different pre-input stage circuits to reduce high-voltage input levels to low-voltage signals. The designs are based on stacked low-voltage standard CMOS transistors.

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