Analog input protection for high-performance signal chains is often a headache for system designers. Often, there is a trade-off between analog performance (such as leakage and on-resistance) and the level of protection (which can be provided by discrete devices).
Replacing discrete protection devices with analog switches and multiplexers with overvoltage protection can provide significant advantages in analog performance, robustness, and solution size. Overvoltage protection devices are located between sensitive downstream circuits and externally stressed inputs. An example is a sensor input in a process control signal chain.
This article details the problems caused by overvoltage events, discusses traditional discrete protection solutions and their associated disadvantages, describes the features and system benefits of overvoltage protection analog switch solutions, and concludes with an introduction to ADI’s industry-leading fault protection analog Switch product line.
Overvoltage Problems – Reviewing the Basics
If the input signal applied to the switch exceeds the supply voltage (VDD or VSS) by more than one diode drop, the ESD protection diodes within the IC will become forward biased and current will flow from the input signal to the supply as shown in the figure 1 shown. This current can damage components and, if not limited, can trigger a latch-up event.
Figure 1. Overvoltage current path.
If the switch is not powered up, the following situations can occur:
If the supply is floating, the input signal may power up the VDD rail through the ESD diode. In this case, the VDD level will be within the range of the input signal voltage minus one forward diode drop.
If the power supply is grounded, the PMOS device will turn on at negative VGS, and the switch will pass the clipped signal to the output, potentially damaging downstream devices that are also unpowered (see Figure 2). Note: If there are diodes connected to the power supply, they will be forward biased, clipping the signal to +0.7 V.
Figure 2. Overvoltage signal when power supply is grounded.
Discrete Protection Solutions
Designers often solve input protection problems with discrete protection devices.
Large series resistors are typically used to limit current during faults, while Schottky or Zener diodes connected to the supply rails will clamp any overvoltage signals. Figure 3 shows an example of such a protection scheme in a multiplexed signal chain.
However, there are a number of disadvantages to using such discrete protection devices.
The series resistance increases the settling time of the multiplexer and reduces the overall settling time.
The protection diodes create additional leakage currents and changing capacitances that affect the accuracy and linearity of the measurement results.
There is no protection when the power supply is floating because the ESD diodes connected to the power supply do not provide any clamping protection.
Figure 3. Discrete protection solutions.
Traditional switch architecture
Figure 4 is an overview of a conventional switch architecture. In the switching device (on the right side of Figure 4), the ESD diodes are connected to the supply rails at the input and output of the switching element. Also shown are external discrete protection devices—a series resistor for current limiting and a Schottky diode (connected to the power supply) for overvoltage clamping. In harsh environments, it is often necessary to provide additional protection with a two-way TVS.
Figure 4. Traditional switching architecture with external discrete protection devices.
Fail-Safe Switch Architecture
The failsafe switch architecture is shown in Figure 5. The ESD diodes on the input are replaced with bidirectional ESD cells, and the input voltage range is no longer limited by the ESD diodes connected to the supply rails. Therefore, the voltage at the input may reach the process limit (±55 V for the new fault-protected switch from ADI).
In most cases, the ESD diodes are still present at the output because overvoltage protection is usually not required at the output.
The ESD cell at the input still provides excellent ESD protection. The ADG5412F overvoltage fault protected quad SPST switch using this type of ESD cell has an HBM ESD rating of 5.5kV.
ADG5412F Product Details
Overvoltage protection up to
?55V and +55V
Shutdown protection up to ~55 V and +55 V
SOURCE pin with overvoltage protection
Low on-resistance: 10 ?
On-resistance flatness: 0.5 ?
Human Body Model (HBM) ESD rating: 5.5 kV
Latch-up resistance in all conditions
Known state without digital Display
Analog Voltage Range VSS to VDD
±5 V to ±22 V dual power supply
8 V to 44 V single power supply
Rated supply voltages: ±15 V, ±20 V, +12 V, and +36 V
For more stringent cases such as IEC ESD (IEC 61000-4-2), EFT or surge protection, an external TVS or a small current limiting resistor may still be required.
Figure 5. Failsafe switch architecture.
When an overvoltage condition occurs on one input of the switch, the affected channel will shut down and the input will go to a high-impedance state. The leakage current on the other channels is still small, so the remaining channels can continue to function normally with minimal impact on performance. There is little to no compromise between system speed/performance and overvoltage protection.
Therefore, a failsafe switch can greatly simplify the signal chain solution. Switching overvoltage protection eliminates the need for current limiting resistors and Schottky diodes in many cases. Overall system performance is also no longer limited by external discrete components that typically cause signal chain leakage and distortion.
Features of ADI’s Fault Protected Switches
ADI’s new line of fault-protected switches are built on a proprietary high-voltage process and provide overvoltage protection up to ±55V in both powered and unpowered states. These devices provide industry-leading performance for fault-protected switches used in precision signal chains.
Figure 6. Trench isolation process.
The proprietary high-voltage process also utilizes trench isolation technology. There is an insulating oxide layer between the NDMOS and PDMOS transistors of each switch. Therefore, unlike junction-isolated switches, there are no parasitic junctions between transistors, suppressing latch-up in all cases. For example, the ADG5412F passes the JESD78D latch-up test with a 1-second pulse width of ±500mA, the most stringent test in the specification.
The new ADI fault-protected switches not only achieve industry-leading robustness (overvoltage protection, high ESD rating, known state at power-up without digital input control), but also feature industry-leading analog performance. The performance of an analog switch is always a trade-off between low on-resistance and low capacitance/charge injection. The choice of analog switch usually depends on whether the load is high impedance or low impedance.
low impedance system
Low-impedance systems typically use low on-resistance devices, where the on-resistance of the analog switches needs to be kept to a minimum. In electrical and other low impedance systems – such as a source or gain stage – the on-resistance and source impedance in parallel with the load can cause gain errors. While gain error can be calibrated in many cases, distortion caused by on-resistance (RON) variation within the signal range or between channels cannot be removed by calibration. Therefore, low-resistance circuits are more subject to distortion errors due to RON flatness and channel-to-channel RON variation.
Figure 7 shows the on-resistance characteristics of a novel fault-protected switch over the signal input range. In addition to being able to achieve extremely low on-resistance, RON flatness and channel-to-channel consistency are also excellent. These devices feature a patented switching driver design that ensures a constant VGS voltage over the signal input voltage range resulting in flat RON performance. The trade-off is that the signal input range is slightly reduced and the switch turn-on performance is optimized, which can be seen from the shape of the RON plot. In applications that are sensitive to RON changes or THD, this RON performance can give the system a distinct advantage.
Figure 7. Fault-protected switch on-resistance.
The ADG5404F is a new type of multiplexer with latch-up proof, overvoltage fault protection. Devices with latch-up immunity and overvoltage protection typically have higher on-resistance and poorer on-resistance flatness than standard devices. However, due to the constant VGS scheme used in the ADG5404F design, the RON flatness is actually better than the ADG1404 (industry-leading low on-resistance) and the ADG5404 (latch-up proof, but without overvoltage protection). In many applications, such as RTD temperature measurement, RON flatness is actually more important than the absolute value of on-resistance, so fault-protected analog switches have the potential to improve their product performance in such systems. A typical failure mode for a low impedance system is that the drain output becomes open circuit in the event of a failure.
High impedance system
Low leakage current, low capacitance, and low charge injection switches are commonly used in high impedance systems. Data acquisition systems typically have high impedance due to amplifier loading on the multiplexer output.
Leakage current is a major source of error in high impedance circuits. Any leakage current can cause significant measurement errors.
Low capacitance and low charge injection are also critical for fast settling. This allows the data acquisition system to achieve maximum data throughput.
The leakage performance of the new ADI fault-protected switches is excellent. During normal operation, the leakage current is in the low nA range, which is critical for accurate measurements in many applications. Best of all, even if one of the input channels fails, the leak-proof performance is excellent. This means that other channels can continue to be measured until the fault is repaired, reducing system downtime. The overvoltage leakage current of the ADG5248F8:1 multiplexer is shown in Figure 8.
A typical failure mode for high-impedance systems is to have the drain output pulled to the supply rail in the event of a failure.
Figure 8. Temperature characteristics of ADG5248F overvoltage leakage current.
Most new ADI fault-protected switches also feature a digital fault pin. The FF pin is a general fault flag, indicating that one of the input channels is in a fault state. A special fault pin (or SF pin) can be used to diagnose which particular input is in a fault state.
These pins are useful for troubleshooting in the system. The FF pin first warns the user of a fault. The user can then poll the digital input and the SF pin will report which particular switch or channel is in a fault state.
Figure 9 shows the system benefits of the new product family of failsafe switches. The benefits of this product family to system designers are enormous, both in terms of ensuring excellent analog performance in precision signal chains, and in terms of system robustness.
Figure 9. ADI Fault Safe Switch—Features and System Benefits.
Compared with discrete protection devices, the advantages are very obvious, and these advantages have been described in detail earlier. The proprietary high-voltage process and new switch architecture also give ADI’s new family of fault-protected switches several advantages over competing solutions.
Industry-leading RON flatness ideal for precision measurements
Industry-leading fault leakage current to continue operation on other channels unaffected by faults (10x better performance than comparable solutions)
Device features secondary fault power supply for precise fault threshold setting while maintaining excellent analog switching performance
Smart Fault Signs for System Troubleshooting
The first application example shown in Figure 10 is a process control signal chain, where a microcontroller can monitor multiple sensors, such as RTD or thermocouple temperature sensors, pressure sensors, and humidity sensors. In a process control application, a sensor may be connected to a very long cable in a factory, and the entire cable may fail.
The multiplexer used in this example is the ADG5249F, which is optimized for low capacitance and low leakage current. Low leakage current is very important for such small signal sensor measurement applications.
The analog switch uses ±15V power supply, and the secondary fault power supply is set to 5V and GND, which can protect the downstream PGA and ADC.
The main sensor signal is routed through the multiplexer to the PGA and ADC, while the fault diagnostic information is sent directly to the microcontroller to provide an interrupt function in the event of a fault. Thus, the user can be alerted to fault conditions and determine which sensors are failing. A technician can then be dispatched to debug the fault and, if necessary, replace the faulty sensor or cable.
Thanks to the industry’s advanced low fault leakage current specification, when one sensor fails and is awaiting replacement, the other sensors can continue to perform monitoring functions. Without this low fault leakage current, a failure of one channel could render all other channels unusable until the fault is repaired before re-use.
Figure 10. Process control application example.
The second application example in Figure 11 is part of a data acquisition signal chain where the ADG5462F channel protector adds additional value. In this example, the PGA is powered by ±15V, while the downstream ADC has an input signal range of 0V to 5V.
The channel protector is located between the PGA and ADC. The main supply is ±15V for excellent on-resistance performance, while the secondary rails are 0V and 5V. During normal operation, the ADG5462F allows signals to pass, but clamps all overvoltage outputs of the PGA to between 0V and 5V to protect the ADC. Therefore, as in the previous application example, the target signal input range will be in the flat RON operating region.
Figure 11. Example of a data acquisition application.
Replacing traditional discrete protection devices with analog switches and multiplexers with overvoltage protection provides several system benefits in precision signal chains. In addition to saving board space, the performance benefits of replacing discrete components are also significant.
Analog Devices offers a variety of analog switches and multiplexers with overvoltage protection. Manufactured using a proprietary high-voltage and latch-up-proof process, this family of products provides industry-leading performance and features for precision signal chains.