AD7887: Using the interal reference

Question I have used AD converter AD7887 in analog acquisition board and I have found this problem: I have used on-chip reference externaly on board and reference voltage change its value when conversions are stopped. I use AD7887 in mode 1 (PM1=0, PM0=0), pin DIN is tied permanently low and AIN1/Vref is buffered by opamp before it is used on board. Is the reference voltage influenced by powering down part via the CS# pin ? Is it necesary tied CS# pin permanently low, when I want to use internal reference externaly (if I want to use mode 1 for simplicity) or is necesary to use mode 2 or mode 4? Answer Yes, mode2 or 4 is necessary if you want to use the internal reference of the AD7887 asa reference source on your board. I would not recommend the use of Mode 1, as this mode expects a falling edge of CS to power up the part. Even though we don’t specify that the internal ref is powered down in mode 1, we hint at it when we describe mode 4.

AD7887: Overvoltage protection

Question Our project uses the AD7887 in a mixed voltage system. During a fault / failure it is possible that the component is not supplied with 5V, but that one of it's inputs (CS and/or CLK) is set high by a 3.3V component on the PCB. I presume there are protection diodes between the chip's inputs and the supply voltage rails within the chip. What is the maximum current that can flow through these diodes? Answer In general you should not rely on internal ESD protection diodes to protect against fault conditions. ESD hits are not the same as a sustained overvoltage condition. If you are likely to see the fault condition you describe, I suggest you add external Schottky diodes and current limiting resistor to CS, CLK and any other input which is likely to exceed the abs max ratings. Note the diodes need to be mounted physically close to the AD7887. Ideally, you should design your system to ensure that the AD7887 is power up fully before any analog or digital signals are applied. I've also included a few general notes on overvoltage. Any other semiconductor IC has basic ESD protection diodes which protect the device from possible ESD hits due to handling and production. It is the designers responsibility to provide external protection circuitry if the input is likely to exceed the supplies at any time. These ESD diodes can protect the IC from ESD hits up to about 1.5kV. These ESD protection diodes will act to clamp the voltage at any pin to within 0.5V of the supplies. (So that's the problem solved right? No not quite.) ESD protection diodes can carry quite high currents but only for a short period of time so they can protect the IC from large pulses of short duration (the total energy is still quite low). The maximum DC current which these protection diodes can carry is 10mA. Therefore unless you can guarantee that the current into in pin will me less then 10mA you need some kind of external protection. External protection could be as simple as a series resistor to limit the current into a pin. For example if the maximum overvoltage voltage applied to a pin will be 5V you need to add a 500Ohm series resistor in each digital line to limit the current to better. A high series resistance in a digital IO line can cause other problems such as slowing the rise and fall time of high speed digital signals. If connections will be accessible by the user, you'll also need to consider ESD protection to much higher levels than +-1.5kV. You might also want to protect against higher overvoltages but you don't want to add any more series resistance, so what can you do? Well, you can do this by adding external Schottky diodes between each digital IO line and the supply lines. A schottky diode will clamp applied voltages to within ~0.3V of the supply so the majority of the current will be diverted via the external diodes (which can carry higher current) and not through the internal ESD protection diodes. There are other protection techniques which include use of spark gaps, large capacitors to earth ground, small choke inductors and more. One the best structures I have seen for protecting against both overvoltage and ESD is a small series resistor followed by Schottky diodes to the supplies followed by another small series resistor. You can see that designing suitable protection circuitry is not a trivial matter. You need to decide how much protection you need, how much abuse you expect the card to be subjected to, how much board space and component cost you can allow, and what test levels you need to meet. Check app notes AN202 and AN397 for more info.

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The AD7887 is a high speed, low power, 12-bit analog-to-digital converter (ADC) that operates from a single 2.7 V to 5.25 V power supply. The AD7887 is...
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- AD7887: Overvoltage protection
- AD7887: Using the interal reference
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