In industrial applications using digital-to-analog converters (DACs), programmable logic controllers (PLCs) set an analog output voltage to control actuators, motors, and valves. PLCs can also regulate manufacturing parameters such as temperature, pressure, and flow.

In these environments, the DAC output may require overvoltage protection from accidental shorts to higher-voltage power supplies and other sustained high-voltage miswired connections. You can protect precision DAC outputs in two different ways, depending on whether the DAC output buffer has an external feedback pin.

Overvoltage damage

There are two potential consequences should an accidental sustained overvoltage event occur at the DAC output.

First, if the DAC output can drive an unsustainable current limit, then damage may occur as the output buffer drives an excess of current. This current limit may also occur if the output voltage is shorted to ground or to another voltage within the supply range of the DAC.

Second, electrostatic discharge (ESD) diodes latched to the supply and ground can source and sink current during sustained overvoltage events, as shown in Figure 1 and Figure 2. In many DACs, a pair of internal ESD diodes that shunt any momentary ESD current away from the device can help protect the output pin. In Figure 1, a large positive voltage causes an overvoltage event in the output and forward-biases the positive AVDD ESD diode. The VOUT pin sinks current from the overvoltage event into the positive supply.

Figure 1 Current is shunted to positive supply during a positive overvoltage event. Source: Texas Instruments

In Figure 2, the negative overvoltage sources current from the negative supply through the AVSS ESD diode to VOUT.

Figure 2 Current is shunted to positive supply during a negative overvoltage event. Source: Texas Instruments

In Figure 1 and Figure 2, internal ESD diodes are not designed to sink or source current associated with a sustained overvoltage event, which will typically damage the ESD diodes and voltage output. Any protection should limit this current during an overvoltage event.

Overvoltage protection

While two basic components will protect precision DAC outputs from an overvoltage event, the protection topology for the DAC depends on the internal or external feedback connection for the DAC output buffer.

If the DAC output does not have an external voltage feedback pin, you can set up protection as a basic buffer using an operational amplifier (op amp) and a current protection device at its output. If the DAC has an external voltage feedback pin, then you would place the current protection device at the output of the DAC, with the op amp driving the feedback sense pin.

Let’s explore both topologies.

Figure 3 shows protection for a DAC without a feedback sense pin, with the op amp set up as a unity gain buffer. Inside the op amp feedback, an eFuse opens the circuit if the op amp output current exceeds a set level.

Figure 3 Output protection for a DAC works without a feedback pin. Source: Texas Instruments

Again, if the output terminal voltage is within the supplies of the op amp, the output current comes from the short-circuit current limit. An output terminal set beyond the supplies of the op amp, as in a positive or negative overvoltage, will cause the supply rails to source or sink additional current, as previously shown in Figure 1 and Figure 2.

Because the output terminal connects to the op amp’s negative input, the op amp input must have some sort of overvoltage protection. For this protection circuit, an op amp with internal overvoltage protection that extends far beyond the op amp supply voltage is selected. When using a different op amp, series resistance that limits the input current can help protect the inputs.

The circuit shown in Figure 3 will also work for a precision DAC with a feedback sense pin. The DAC feedback sense pin would simply connect to the DAC VOUT pin, using the same protection buffer circuit. If you want to use the DAC feedback to reduce errors from long output and feedback sense wire resistances, you need to use a different topology for the protection circuit.

If the DAC has an external feedback sense pin, changing the protection preserves the sense connection. In Figure 4, the eFuse connects directly to the DAC output. The eFuse opens if the DAC output current exceeds a set level. Here, the op amp acts as a unity gain buffer to drive the DAC sense feedback pin.

Figure 4 This output protection for a DAC works with a feedback pin. Source: Texas Instruments

In both topologies, shown in Figure 3 and Figure 4, the two protection devices have the same requirements. For the eFuse, the break current must be lower than the current level that might damage the device it’s protecting. For the op amp, input protection is required, as the output overvoltage may significantly exceed the rail voltage. In operation, the offset voltage must be lower than the intended error, and the bandwidth must be high enough to satisfy the system requirements.

Overvoltage protection component selection

To help you select the required components, here are the system requirements for operation and protection:

• Supply range: ±15 V
• Sustained overvoltage protection: ±32 V
• Current at sustained overvoltage: approximately 30 mA
• Output protection should introduce as little error as possible, based on offset or bandwidth

The primary criteria for op amp selection were overvoltage protection of the inputs. For instance, the super-beta inputs of the OPA206 precision op amp have an integrated input overvoltage protection that extends up to ±40 V beyond the op amp supply voltage. Figure 5 shows the input bias current relative to the input common-mode voltage powering OPA206 with ±15 V supplies. Within the ±32 V range of overvoltage protection, the input bias current stays below ±5 mA of input current.

Figure 5 Input bias current for the OPA206 is shown versus the input common-mode voltage. Source: Texas Instruments

The OPA206 offset voltage is very low (typically ±4 µV at 25°C and ±55 µV from –40°C to 125°C) and the buffer contributes little error to the DAC output. When using a different op amp without integrated input overvoltage protection, adding series resistance at the inputs will limit the input current.

The TPS2661 eFuse was originally intended as a current-loop protector with input and output miswiring protection. If its output voltage exceeds the rail supplies, TPS2661 detects miswiring and cuts off the current path, restoring the current path when the output overvoltage returns below the supply.

If the output current exceeds TPS2661’s 32-mA current-limit protection, the device breaks the connection and retests the current path for 100 ms periodically every 800 ms. The equivalent resistance of the device is a maximum 12.5 Ω, which enables a high-current transmission output without large voltage headroom and footroom loss at the output.

Beyond the op amp and eFuse protection, applying an optional transient voltage suppression (TVS) diode will provide additional surge protection as long as the chosen breakdown voltage is higher than any sustained overvoltage. If the breakdown voltage is less than the sustained overvoltage, then an overvoltage can damage the TVS diode. In this circuit, the expected sustained overvoltage is ±32 V, with an optional TVS3301 device that has a bidirectional 33-V breakdown for surge protection.

Another TVS3301 added to the ±15-V supplies is an additional option. An overvoltage on the terminal will direct any fault current into the power supplies. If the supply cannot sink the current or is not fast enough to respond to the overvoltage, then the TVS diode absorbs excess current as the overvoltage occurs.

Constructed circuit: Precision DAC without a feedback sense pin

You can build and test the overvoltage protection buffer from Figure 3 with the DAC81416-08 evaluation module (EVM). This multichannel DAC doesn’t have an external feedback sense pin. Figure 6 shows the constructed protection buffer tested on one of the DAC channels.

Figure 6 The constructed overvoltage protection circuit employs the DAC81416-08 evaluation module. Source: Texas Instruments

Ramping the output of DAC from –10 V to 10 V drives the buffer input. Figure 7 shows that the measured offset of the buffer is less than 10 µV over the full range.

Figure 7 Protection buffer output offset error is shown versus buffer input voltage. Source: Texas Instruments

Connecting the output to a variable supply tests the output overvoltage connection, driving the output voltage and then recording the current at the output. The measurement starts at –32 V, increases to +32 V, then changes back from +32 V down to –32 V. Figure 8 shows the output current set to overvoltage and its recovery from overvoltage.

Figure 8 Protection buffer output current is shown versus buffer output overvoltage. Source: Texas Instruments

The measurements show hysteresis in both the positive and negative overvoltage of the protection buffer that comes from extra voltage across the series resistor at the output of the TPS26611. During normal operation (without an overvoltage), the TPS26611 current path turns off when the output rises and is driven above 17.2 V, at which point the remaining output current comes from the overvoltage of the OPA206 input. As the output voltage decreases, the TPS26611 current path conducts current again when the output drops below 15 V.

When driving the output to a negative overvoltage, the current path turns off at –17.5 V and turns on again when the output returns above –15 V.

Constructed circuit: Protection for a DAC with output feedback

Like the previous circuit, you can test the overvoltage protection from Figure 4. This test attaches an overvoltage protection buffer to the output of a DAC with an external feedback sense pin. The DAC8760 EVM tests for an output overvoltage event. As shown in Figure 9, a 1-kΩ resistor placed between VOUT and +VSENSE prevents the output buffer feedback loop of the DAC from breaking if the feedback sense signal is cut.

Figure 9 This constructed overvoltage protection circuit is used with the DAC8760 evaluation module. Source: Texas Instruments

Ramping the output of the DAC from –10 V to +10 V drives the feedback buffer input. Shown in Figure 10, the offset of the feedback to +VSENSE is again <10 μV over the full range.

Figure 10 Feedback buffer offset error is shown versus buffer input voltage. Source: Texas Instruments

The DAC is again set to 0 V, with the output connected to a variable supply to check the output current against output overvoltage. Figure 11 shows the output current as the output voltage increases from –32 V to +32 V and decreases to –32 V.

Figure 11 Protection buffer output current is shown versus buffer output overvoltage. Source: Texas Instruments

As before, there is current path hysteresis. The TPS26611 current path shuts off when the output goes above 16.5 V and turns on when the output returns to about 15 V. For the negative overvoltage, the current path turns off when the output is below –16.8 V and turns on again when the output returns above –15 V.

Two overvoltage protection topologies

Industrial control applications for analog outputs require specialized protection from harsh conditions. This article presented two topologies for precision DAC protection against sustained overvoltage events:

• DAC without external feedback: Protecting the output from an overvoltage by using an op amp buffer with an eFuse in the op amp output.
• DAC with external feedback: Protecting the output from overvoltage by using an eFuse to limit the DAC output current and with an op amp acting as a unity gain buffer for sense feedback.

In both cases, the tested circuits show a limited offset error (<10 µV) through the range of operation (±10-V output) and protection from sustained overvoltage of ±32 V.

Joseph Wu is applications engineer for digital-to-analog converters (DACs) at Texas Instruments.

 

 

Art Kay is applications engineer for precision signal conditioning products at Texas Instruments.

 

 

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