by Daniel Burton
High-precision op amps enable system designers to create circuits that condition signals (amplify, filter, buffer, etc.) while maintaining the precision of the original signal. When information is contained in very small variations of the signal, it is critical that op amps in the signal path perform their operation while contributing very little DC and AC error. The performance of the total system depends on maximizing the precision and accuracy of the original signal throughout the path.
In some applications, a situation may occur in which the inputs of the op amp get driven by voltages outside the level of the supply voltages — called an over-voltage condition. For example, if an op amp is configured to run with its positive supply at +15V and its negative supply at -15V, any time an input pin goes more than one diode drop beyond those supply rails (say, ±15.7V), the op amp’s internal ESD-protection diodes can be forward biased and start conducting current. Excessive input current over long periods of time (or even short periods of time if the current is high enough) can damage the op amp. This damage can result in a shift in the electrical specification parameters beyond the datasheet guaranteed limits; it can even cause a permanent failure of the op amp. When system designers are faced with this possible situation, they often add over-voltage protection (OVP) circuits at the inputs to the amplifier. The challenge then is to add OVP circuitry without adding errors (loss of system precision).
How Over-Voltage Conditions Occur
Over-voltage conditions can be caused by a number of different situations. Consider a system where a remote sensor is located in the field (for example, measuring fluid flow in a refinery) and sending its signal through a cable to data-acquisition electronics which reside at a different physical location. The first stage in the data-acquisition electronics signal path can often be an op amp configured as a buffer or a gain amplifier. The input to that op amp is exposed to the outside world and therefore can be subjected to any over-voltage incidents like a short circuit from a damaged cable or incorrectly connecting the cable to the data-acquisition electronics.
Similarly, a situation that can cause an over-voltage condition is when an input signal that is usually within the input voltage range of the amplifier suddenly receives an external stimulus which causes a transient spike that exceeds the op amp supply voltages.
A third scenario that can result in an input over-voltage condition comes from the power-on sequence of the op amp and other components in the signal path. For example, if the signal source (e.g., a sensor) gets powered up before the op amp does, the output of the source can start to output a voltage that will then be applied to the input of the op amp even though the op amp supply pins have no power yet (they are essentially at ground). This will create an over-voltage situation and likely force excessive current through the input of the op amp to ground (the unpowered supply pins).
Clamping: A Classic Over-Voltage Protection Technique
A very popular way to add OVP is shown in Figure 1. When the amplitude of the input signal (VIN) exceeds one of the supply voltages plus the forward voltage of a diode, the diode (DOVPP or DOVPN) will forward bias and send the current to the supply rails rather than into the op amp inputs, where the excess current could damage the op amp. In this application, we are using an ADA4077, an extremely high-precision and low input-offset op amp with a maximum power supply range of 30V (or ±15V).
The clamping diodes are 1N5177 Schottky diodes because they have a forward voltage of approximately 0.4V, which is less than the forward voltage of the op amp’s input electro-static discharge (ESD) protection diodes; thus the clamping diodes will start conducting current before the ESD diodes do. The over-voltage protection resistor ROVP limits the forward current through the clamping diodes to keep them under their maximum current rating, preventing them from being damaged by excessive current. The resistor RFB in the feedback loop is there because any input bias current on the non-inverting input can cause an input voltage error across ROVP; adding RFB will null out the error by generating a similar voltage on the inverting input.
The Trade-off of a Diode Clamping Circuit: Reduced Precision
Although the classic circuit in Figure 1 does protect the op amp inputs, it contributes a significant amount of error to the signal path. High-precision amps generally have low input offset voltages (VOS) in the microvolts range. For example, the maximum VOS for an ADA4077 is 35µV over the full operating temperature range of -40°C to 125°C. Adding the external diodes and an overvoltage resistor contributes an input offset error that can be many times greater than the low offset inherent to the op amp.
Reverse-biased diodes exhibit a reverse leakage current which flows from the cathode through the anode to the supply. When the input signal voltage (VIN) is between the supply rails, the diodes DOVPN and DOVPN have a reverse voltage on them. With VIN at ground (the middle of the input voltage range), the reverse current though DOVPN is approximately equal to the reverse leakage current through DOVPP. However, when VCM moves above or below ground, a larger reverse current flows through one diode than the other. For example, when VCM is at the top of the op amp’s input voltage range (which is 2V from the positive supply or +13V in this circuit), diode DOVPN will have a reverse voltage of 28V across it. According to the 1N5177 diode datasheet, this can cause a reverse leakage current of close to 100 nA. As reverse-leakage current flows from the input signal (VIN) through ROVP, it will create a voltage drop across ROVP which looks to the signal path exactly like an increased input offset voltage.
Of additional concern is that diode reverse leakage current increases exponentially with an increase in temperature — causing an increase in the offset-voltage penalty of the clamping OVP circuit. As a baseline of comparison for op amp precision with no external over-voltage circuitry, Figure 2 shows the measured offset voltage of the ADA4077 over an input voltage range from -13V to +13V. The measurements were performed at three temperatures: 25°C, 85°C, and 125°C. Note that at 25°C, the VOS of the ADA4077 used in this test reached only 6 microvolts; even at 125°C, the VOS is only approximately 20µV. When we add the external clamping OVP circuit to the same ADA4007 device and apply the input at VIN, we see the results shown in Figure 3. At room temperature, the VOS jumps to 30 microvolts — five times the signal path error of the ADA4077 alone. At 125°C, VOS goes to over 15 millivolts — an increase 750 times the 20 µV of the ADA4077! The precision is gone.
The 5kΩ resistor does a great job protecting the clamping diodes (and therefore the op amp) during an over-voltage condition, but it adds quite a bit of offset error — reducing precision — during normal operation when the diodes are leaking current across it (not to mention loss of precision from the Johnson noise of the resistor). What we would like is a “dynamic” input resistance that has low resistance during operation within the specified input voltage range, but high resistance during over-voltage conditions.
An Integrated Solution Provides the Answer
The ADA4177 is a high-precision, low-offset op amp which includes integrated over-voltage protection. The integrated ESD diodes act as overvoltage clamps to protect the part. Depletion-mode FETs are in series at each input before the ESD diodes. They provide the dynamic resistance which increases when the input voltage (VCM) exceeds the supply voltages. As input voltage increases, the drain-to-source resistance (RDSON) of an internal FET increases, thus restricting the current flow exponentially with the increased voltage (Figure 4). Because the ADA4177 uses depletion mode FETs on the inputs and not a series protection resistor, the op amp doesn’t suffer the offset-voltage penalty across the resistor that the clamping OVP circuit does.
The ADA4177 can withstand voltages on its inputs up to 32 Volts beyond the supply voltage. It limits overvoltage current to a typical 10-12 mA, protecting the op amp without the use of any external components. As shown in Figure 5, even at 125°C, this tested unit is showing an offset voltage of only 40 microvolts. That’s less than 3% of the error that the clamping circuit showed at that temperature! Precision is maintained.
What This Means to System Performance
When analyzing the effect of varying input voltage on the precision of the signal path, a system designer will consider the amplifier’s common mode rejection ratio (CMRR). This is a measure of how much of the common-mode input voltage is rejected from showing up on the output (or how little gets through). Since op amps are often configured to provide gain between the input and the output, we normalize the CMRR specification by referring to change in the input offset voltage (the change in output divided by the amplifier closed-loop gain). The common mode rejection ratio is a positive value expressed in dB and is calculated by the following formula:
CMRR = 20 log (ΔVCM /ΔVOS)
From this ratio, we see it is clearly desirable to keep the VOS as small as possible. The ADA4177 is specified to have a guaranteed minimum CMRR limit of 125 dB over full operating temperature. Using the test results from the units measured in this experiment, we can calculate and compare the CMRR of the clamping circuit and the ADA4177. Table 1 shows the extreme loss of precision when using the classic clamping diode circuit and the excellent CMRR of the ADA4177 with its integrated FET over-voltage protection.
For more information on designing high-precision amplifier circuits with over-voltage protection, see:
Robust Amplifiers Provide Integrated Overvoltage Protection by Eric Modica and Michael Arkin, Analog Dialogue Volume 46, February 2012: http://www.analog.com/library/analogDialogue/archives/46-02/ovp.html
Video (ADA4177): Op Amp with OVP and EMI Provides Robustness and Precision: http://bcove.me/dmuw92a5
Video: ADA4096-2 Input Over-Voltage Protection Amplifier: http://bcove.me/bdj41ehl