In industrial purposes utilizing digital-to-analog converters (DACs), programmable logic controllers (PLCs) set an analog output voltage to manage actuators, motors, and valves. PLCs can even regulate manufacturing parameters equivalent to temperature, stress, and movement.
In these environments, the DAC output might require overvoltage safety from unintended shorts to higher-voltage energy provides and different sustained high-voltage miswired connections. You possibly can defend precision DAC outputs in two other ways, relying on whether or not the DAC output buffer has an exterior suggestions pin.
Overvoltage injury
There are two potential penalties ought to an unintended sustained overvoltage occasion happen on the DAC output.
First, if the DAC output can drive an unsustainable present restrict, then injury might happen because the output buffer drives an extra of present. This present restrict might also happen if the output voltage is shorted to floor or to a different voltage throughout the provide vary of the DAC.
Second, electrostatic discharge (ESD) diodes latched to the provision and floor can supply and sink present throughout sustained overvoltage occasions, as proven in Determine 1 and Determine 2. In lots of DACs, a pair of inside ESD diodes that shunt any momentary ESD present away from the system can assist defend the output pin. In Determine 1, a big constructive voltage causes an overvoltage occasion within the output and forward-biases the constructive AVDD ESD diode. The VOUT pin sinks present from the overvoltage occasion into the constructive provide.
Determine 1 Present is shunted to constructive provide throughout a constructive overvoltage occasion. Supply: Texas Devices
In Determine 2, the unfavourable overvoltage sources present from the unfavourable provide by the AVSS ESD diode to VOUT.
Determine 2 Present is shunted to constructive provide throughout a unfavourable overvoltage occasion. Supply: Texas Devices
In Determine 1 and Determine 2, inside ESD diodes are usually not designed to sink or supply present related to a sustained overvoltage occasion, which can sometimes injury the ESD diodes and voltage output. Any safety ought to restrict this present throughout an overvoltage occasion.
Overvoltage safety
Whereas two fundamental elements will defend precision DAC outputs from an overvoltage occasion, the safety topology for the DAC will depend on the interior or exterior suggestions connection for the DAC output buffer.
If the DAC output doesn’t have an exterior voltage suggestions pin, you’ll be able to arrange safety as a fundamental buffer utilizing an operational amplifier (op amp) and a present safety system at its output. If the DAC has an exterior voltage suggestions pin, then you definately would place the present safety system on the output of the DAC, with the op amp driving the suggestions sense pin.
Let’s discover each topologies.
Determine 3 exhibits safety for a DAC and not using a suggestions sense pin, with the op amp arrange as a unity acquire buffer. Contained in the op amp suggestions, an eFuse opens the circuit if the op amp output present exceeds a set stage.
Determine 3 Output safety for a DAC works and not using a suggestions pin. Supply: Texas Devices
Once more, if the output terminal voltage is throughout the provides of the op amp, the output present comes from the short-circuit present restrict. An output terminal set past the provides of the op amp, as in a constructive or unfavourable overvoltage, will trigger the provision rails to supply or sink extra present, as beforehand proven in Determine 1 and Determine 2.
As a result of the output terminal connects to the op amp’s unfavourable enter, the op amp enter should have some form of overvoltage safety. For this safety circuit, an op amp with inside overvoltage safety that extends far past the op amp provide voltage is chosen. When utilizing a distinct op amp, collection resistance that limits the enter present can assist defend the inputs.
The circuit proven in Determine 3 may also work for a precision DAC with a suggestions sense pin. The DAC suggestions sense pin would merely hook up with the DAC VOUT pin, utilizing the identical safety buffer circuit. If you wish to use the DAC suggestions to cut back errors from lengthy output and suggestions sense wire resistances, it is advisable use a distinct topology for the safety circuit.
If the DAC has an exterior suggestions sense pin, altering the safety preserves the sense connection. In Determine 4, the eFuse connects on to the DAC output. The eFuse opens if the DAC output present exceeds a set stage. Right here, the op amp acts as a unity acquire buffer to drive the DAC sense suggestions pin.
Determine 4 This output safety for a DAC works with a suggestions pin. Supply: Texas Devices
In each topologies, proven in Determine 3 and Determine 4, the 2 safety gadgets have the identical necessities. For the eFuse, the break present should be decrease than the present stage that may injury the system it’s defending. For the op amp, enter safety is required, because the output overvoltage might considerably exceed the rail voltage. In operation, the offset voltage should be decrease than the meant error, and the bandwidth should be excessive sufficient to fulfill the system necessities.
Overvoltage safety part choice
That can assist you choose the required elements, listed below are the system necessities for operation and safety:
- Provide vary: ±15 V
- Sustained overvoltage safety: ±32 V
- Present at sustained overvoltage: roughly 30 mA
- Output safety ought to introduce as little error as potential, primarily based on offset or bandwidth
The first standards for op amp choice had been overvoltage safety of the inputs. As an example, the super-beta inputs of the OPA206 precision op amp have an built-in enter overvoltage safety that extends as much as ±40 V past the op amp provide voltage. Determine 5 exhibits the enter bias present relative to the enter common-mode voltage powering OPA206 with ±15 V provides. Inside the ±32 V vary of overvoltage safety, the enter bias present stays under ±5 mA of enter present.
Determine 5 Enter bias present for the OPA206 is proven versus the enter common-mode voltage. Supply: Texas Devices
The OPA206 offset voltage may be very low (sometimes ±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 utilizing a distinct op amp with out built-in enter overvoltage safety, including collection resistance on the inputs will restrict the enter present.
The TPS2661 eFuse was initially meant as a current-loop protector with enter and output miswiring safety. If its output voltage exceeds the rail provides, TPS2661 detects miswiring and cuts off the present path, restoring the present path when the output overvoltage returns under the provision.
If the output present exceeds TPS2661’s 32-mA current-limit safety, the system breaks the connection and retests the present path for 100 ms periodically each 800 ms. The equal resistance of the system is a most 12.5 Ω, which allows a high-current transmission output with out massive voltage headroom and footroom loss on the output.
Past the op amp and eFuse safety, making use of an non-compulsory transient voltage suppression (TVS) diode will present extra surge safety so long as the chosen breakdown voltage is greater than any sustained overvoltage. If the breakdown voltage is lower than the sustained overvoltage, then an overvoltage can injury the TVS diode. On this circuit, the anticipated sustained overvoltage is ±32 V, with an non-compulsory TVS3301 system that has a bidirectional 33-V breakdown for surge safety.
One other TVS3301 added to the ±15-V provides is a further choice. An overvoltage on the terminal will direct any fault present into the facility provides. If the provision can’t sink the present or isn’t quick sufficient to reply to the overvoltage, then the TVS diode absorbs extra present because the overvoltage happens.
Constructed circuit: Precision DAC and not using a suggestions sense pin
You possibly can construct and take a look at the overvoltage safety buffer from Determine 3 with the DAC81416-08 analysis module (EVM). This multichannel DAC doesn’t have an exterior suggestions sense pin. Determine 6 exhibits the constructed safety buffer examined on one of many DAC channels.
Determine 6 The constructed overvoltage safety circuit employs the DAC81416-08 analysis module. Supply: Texas Devices
Ramping the output of DAC from –10 V to 10 V drives the buffer enter. Determine 7 exhibits that the measured offset of the buffer is lower than 10 µV over the complete vary.
Determine 7 Safety buffer output offset error is proven versus buffer enter voltage. Supply: Texas Devices
Connecting the output to a variable provide assessments the output overvoltage connection, driving the output voltage after which recording the present on the output. The measurement begins at –32 V, will increase to +32 V, then adjustments again from +32 V all the way down to –32 V. Determine 8 exhibits the output present set to overvoltage and its restoration from overvoltage.
Determine 8 Safety buffer output present is proven versus buffer output overvoltage. Supply: Texas Devices
The measurements present hysteresis in each the constructive and unfavourable overvoltage of the safety buffer that comes from further voltage throughout the collection resistor on the output of the TPS26611. Throughout regular operation (with out an overvoltage), the TPS26611 present path turns off when the output rises and is pushed above 17.2 V, at which level the remaining output present comes from the overvoltage of the OPA206 enter. Because the output voltage decreases, the TPS26611 present path conducts present once more when the output drops under 15 V.
When driving the output to a unfavourable overvoltage, the present path turns off at –17.5 V and activates once more when the output returns above –15 V.
Constructed circuit: Safety for a DAC with output suggestions
Just like the earlier circuit, you’ll be able to take a look at the overvoltage safety from Determine 4. This take a look at attaches an overvoltage safety buffer to the output of a DAC with an exterior suggestions sense pin. The DAC8760 EVM assessments for an output overvoltage occasion. As proven in Determine 9, a 1-kΩ resistor positioned between VOUT and +VSENSE prevents the output buffer suggestions loop of the DAC from breaking if the suggestions sense sign is minimize.
Determine 9 This constructed overvoltage safety circuit is used with the DAC8760 analysis module. Supply: Texas Devices
Ramping the output of the DAC from –10 V to +10 V drives the suggestions buffer enter. Proven in Determine 10, the offset of the suggestions to +VSENSE is once more <10 μV over the complete vary.
Determine 10 Suggestions buffer offset error is proven versus buffer enter voltage. Supply: Texas Devices
The DAC is once more set to 0 V, with the output related to a variable provide to verify the output present in opposition to output overvoltage. Determine 11 exhibits the output present because the output voltage will increase from –32 V to +32 V and reduces to –32 V.
Determine 11 Safety buffer output present is proven versus buffer output overvoltage. Supply: Texas Devices
As earlier than, there may be present path hysteresis. The TPS26611 present path shuts off when the output goes above 16.5 V and activates when the output returns to about 15 V. For the unfavourable overvoltage, the present path turns off when the output is under –16.8 V and activates once more when the output returns above –15 V.
Two overvoltage safety topologies
Industrial management purposes for analog outputs require specialised safety from harsh circumstances. This text offered two topologies for precision DAC safety in opposition to sustained overvoltage occasions:
- DAC with out exterior suggestions: Defending the output from an overvoltage by utilizing an op amp buffer with an eFuse within the op amp output.
- DAC with exterior suggestions: Defending the output from overvoltage by utilizing an eFuse to restrict the DAC output present and with an op amp appearing as a unity acquire buffer for sense suggestions.
In each circumstances, the examined circuits present a restricted offset error (<10 µV) by the vary of operation (±10-V output) and safety from sustained overvoltage of ±32 V.
Joseph Wu is purposes engineer for digital-to-analog converters (DACs) at Texas Devices.
Artwork Kay is purposes engineer for precision sign conditioning merchandise at Texas Devices.
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