Shutdown-Protected Data Acquisition Signal Chain Using Fail-Safe CMOS Switches

Circuit functions and advantages

With remote sources, there is a greater chance of damaging failures. Overvoltage may be caused by improperly designed system power sequencing or by system requiring hot swapping. If unprotected, transient voltages caused by poor connections or inductive coupling can damage components. Also, faults can occur when the power supply fails or the switch input is still connected to the analog signal and the power connection is lost. These failure conditions can cause significant damage and the result can mean high repair costs.

The circuit shown in Figure 1 utilizes a low on-resistance, four-channel, single-pole, single-throw switch ADG4612 with power-off protection to provide the Provide protection. The data acquisition system consists of a low cost, precision JFET input op amp ADA4000-1 followed by a low power, 12-bit, 1 MSPS SAR ADCAD7476. The ADG4612 provides low-cost power loss protection and overvoltage fault protection up to 16V when the input signal is still present. This package requires little additional board area. The ADG4612 provides protection for four independent data acquisition channels without adding any discrete components.

Shutdown-Protected Data Acquisition Signal Chain Using Fail-Safe CMOS Switches

Figure 1. Fail-Safe Data Acquisition Signal Chain Circuit (Simplified Schematic: All Connections and Decoupling Not Shown)

circuit description

Figure 1 shows a single-channel, fault-protected data acquisition signal chain consisting of the ADG4612, ADA4000-1, and AD7476. The key to protecting the data acquisition board lies in the protection function of the ADG4612. When there is no power supply, that is, when VDD is floating or VDD ≤ 1 V, or when the input signal VS or VD is greater than the sum of the power supply VDD and the threshold voltage (VT), the switch is in isolation mode. Under these conditions, the switch input is a high impedance input to ensure that no current exists that could damage the switch or downstream circuitry. The negative rail, VSS, can be left floating or 0 V to −5.5 V. The ground pin must be connected to ground potential. Inputs to downstream components, such as the ADA4000-1 or AD7476, should be limited to the supply rails to block these signals when power is lost or if the input signals exceed the supply rails.

In the disconnected condition, input signal levels up to +16V are blocked (assuming VSS = 0 V). The switch also opens when the analog input signal level exceeds VDD by the threshold voltage VT (~1.2 V).

For standard CMOS analog switches, please refer to the product data sheet for their rated power requirements and should be strictly followed to ensure that the device maintains optimum performance and operation. However, due to power failures, voltage transients, improper timing control, system failures, or user failures, it is not always possible to meet data sheet requirements.

Standard CMOS switch” title=”CMOS switch”>The signal source, drain, and logic pins of the CMOS switch provide ESD protection in the form of power clamp diodes, as shown in Figure 2. The dimensions of these diodes vary by process , but generally a small design is used to minimize leakage current. In normal operation, these diodes are reverse biased and will not pass current. When forward biased, their rated specifications require that the current passing through is not greater than a few mA, Failure to do so may damage the device. Whenever the analog switch input voltage exceeds the power supply, these diodes will turn forward biased, potentially passing a higher current, so the switch may be damaged even if the power is turned off. Also, damage from a fault is not Switching is limited, but can also affect downstream circuits, such as the ADA4000-1, because applying a signal to an unpowered ADA4000-1 exceeds the absolute maximum ratings of the device.

Shutdown-Protected Data Acquisition Signal Chain Using Fail-Safe CMOS Switches

Figure 2. ESD protection on a standard analog CMOS switch

Figure 3 shows the performance waveforms of a standard analog switch with a signal applied and the power supply floating. A 6 V pp sine wave with a DC bias of +3 V is applied to the analog input, which, through an internal ESD diode, supplies power to the switch and any other components connected to the same VDD supply. The input signal passes through the switch and appears at the input of the ADA4000-1, thus exceeding the maximum ratings of the ADA4000-1.

Shutdown-Protected Data Acquisition Signal Chain Using Fail-Safe CMOS Switches

Figure 3. Standard analog switch without power

Another limiting factor for standard CMOS switches is that when the analog signal exceeds the power supplies VDD and VSS, the power supply is pulled within the diode drop of the fault signal. The internal diode is forward biased and current flows from the input signal to the power supply. Fault signals can also pass through the switch and damage downstream components, as shown in Figure 4.

Long-term reliability may be affected if the absolute maximum ratings of the device are exceeded.

Shutdown-Protected Data Acquisition Signal Chain Using Fail-Safe CMOS Switches

Figure 4. Standard Analog Switch Under Overvoltage Condition

The ADG4612 eliminates the effects of these faults by using no internal ESD diodes from the analog or digital inputs to VDD or VSS. Instead, the ADG4612 utilizes other protection components to avoid ESD events. This means that under power loss conditions or in the event of an overvoltage fault, there is no low impedance path to the power supply. If a signal is present at the ADG4612 input before power is available, the switch will enter isolated mode, that is, the input has a high impedance path to VDD, GND, and the output. This prevents current flow, effectively protecting the device and downstream circuitry from damage.

Shutdown-Protected Data Acquisition Signal Chain Using Fail-Safe CMOS Switches

Figure 5. ADG4612 without power supply

Figure 6 shows the result of an overvoltage fault on the analog input. At this point, a 6 V pp sine wave with a DC bias of +3 V is applied to the ADG4612, which operates from a ±3.3 V supply. When the analog input exceeds VDD by the threshold voltage VT (~1.2 V), the switch enters isolation mode, thereby preventing faults from causing damage to downstream circuits.

Shutdown-Protected Data Acquisition Signal Chain Using Fail-Safe CMOS Switches

Figure 6. Overvoltage Condition on ADG4612

As shown in Figure 1, the combination of the ADG4612, the ADA4000-1, and the AD7476 provides a robust data acquisition circuit that can effectively cope with various fault conditions, such as power loss faults or analog input while the signal from the outside is still present. terminal overvoltage fault, etc.

Note that the input range of the AD7476 is equal to VDD, which also acts as the reference for this ADC. At this point, the input range is 0 V to +3.3 V. To drive the AD7476 linearly over this range, the supply voltage for the ADA4000-1 must be slightly higher to allow sufficient headroom for the output stage (about 1.2 V with respect to the positive supply and about 2 V with respect to the negative supply). This is accomplished by setting the positive supply voltage of the ADA4000-1 to +5 V and the negative supply voltage to −3.3 V. Two external Schottky diodes connected to the AD7476 inputs ensure that power supply sequencing is not problematic.

The ADG4613 also supports configurations other than SPST; the device has two switches with similar digital control logic to the ADG4612, but the other two switches have the opposite control logic. When switched on, each switch conducts equally in both directions, and the input signal range extends to the supply voltage range. The ADG4613 is a break-before-make switch suitable for multiplexer applications. The device can be configured as a quad SPST, dual SPDT or 4:1 multiplexer to suit different applications.

In order to achieve the desired performance of the circuits discussed in this article, excellent layout, grounding, and decoupling techniques are required. At least four layers of PCB should be used: one is the ground plane, one is the power plane, and the other two are the signal layers.

common changes

Figure 7 shows a variation of the circuit in Figure 1, operating from a single +3.3 V supply. In this application, an op amp with rail-to-rail inputs and outputs is required to drive the full input range of the AD7476. The output of the AD8655 op amp is rated to drive within 10 mV to 30 mV of each supply rail. In other words, there is a small percentage of dead-time encoding at each end of the full-scale ADC range where linearity is adversely affected.

For a headroom requirement of 30 mV, this ratio is approximately 1% of the 3.3V input range. For a detailed discussion of op amp rail-to-rail issues and overvoltage protection, see the MT-035 and MT-036 tutorials.

Also note that the ADG4612 has a VSS of 0 V in this circuit, but still maintains good on-resistance flatness over the entire input signal range.

The AD8656 is a dual-channel version of the AD8655. The ADA4000-2 and ADA4000-4 are two- and four-channel versions of the ADA4000-1, respectively.

Figure 7. Single-Supply, Fail-Safe Data Acquisition Signal Chain Circuit (Simplified Schematic: All Connections and Decoupling Not Shown)

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