AD7982: Driving the reference input

Question I am currently designing a circuit that uses a pulSAR ADC, specifically the AD7982, and am having difficulties understanding the choice of components required for driving the reference pin. The data sheet for the device recommends using a buffer to provide a low impedance source to the reference pin but this is driving a 10uF capacitor. Looking at the data sheets, for the recommended devices, driving such a large capacitance would require a series resistor, or some other unspecified circuit, and would this cause reference sag as detailed in application note AN-931? The choice of the size and type of capacitor is some what confusing. The size of this capacitor seems not to change with the resolution, speed of conversion or for that matter the converter itself. The cause of the settling time problems, outlined in AN931, is also giving me a headache. It refers to a LRC tank circuit where L is the inductance of the reference, C is the storage capacitor and R is the resistance of the switch. I cannot understand how the resistance of the switch has any effect on the resonant frequency of this circuit assuming that it is this that causes the settling time problems? In short 1. Can I use the references detailed in the data sheet to drive a 10uF capacitor without any series resistance? 2. How does the choice of the size and type of capacitance affect the performance? Could I use a tantalum or X7R dielectric for example? 3. What the reasons for the settling time problems? Answer As you have seen in AN931 the reference current of PulSAR ADCs has a mean value of some tens or hundreds of μA during conversions, so series resistance in the reference source will present problems. However, if you use an op-amp buffer you appear to need a series resistance to prevent oscillation or ringing caused by capacitive loading of the amplifier, which causes the same problems, but often the series resistance may be low enough that it does not cause a problem. Series resistors are not the only way of stabilizing op-amps with capacitive loads. If you increase the noise gain (= feedback attenuation) at higher frequencies you can achieve the same effect without the need for a series resistance. Again, this approach limits the signal bandwidth but we are not concerned about signal BW in a reference buffer. The storage or reference input capacitor at the converter reference input must be able to supply fast current pulses without sag. This means it must have low ESL and ESR. Experience shows that capacitors in the range 2.2-22 μF work best. Aluminium electrolytic and ceramic capacitors with significant ESL are unsuitable (not all tantalum capacitors are ideal, either), hence the recommendation to use XR5 - X7R should do equally well. Low ESL is no use if the PC tracks are long and inductive, hence the capacitor MUST be as close as possible to the ADC pins. Analysis of optimal capacitance is not very reliable but experiment may be some use. In general you will not go far wrong using 10 μF. The inductance of the reference refers to the reactive component of the reference output impedance, which is usually inductive. To answer your explicit questions:- [1] The AD780 and the ADR431 will certainly drive 10 μF without problems (but see the ADR43X DS for ways of minimising noise). [2] Inductive capacitors (some plastic film types, many electrolytic and some tantalum ones) are bad news. Low inductance tantalum or, better, ceramic, including X7R, are fine. [3] I hope that my comments above have answered this.

Connection of exposed pad in AD7982 LFCSP package

Q How best to connection the exposed pad on the AD7982 in LFCSP package? A For the LFCSP package, the expose pads is not internally connected to the die. More specifically is for thermal relief. This would fall under the general rules for expose pads. see link to my engineer Zone answers in link at: https://ez.analog.com/message/19558#19558

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The AD7982 is an 18-bit, successive approximation, analog-to-digital converter (ADC) that operates from a single power supply, VDD. The AD7982 contains...
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+Documents
+General: FAQ
+AD1139: FAQ
+AD7091R Conversion Result Change
+AD1556: FAQ
+AD1580: FAQ
+AD1582: FAQ
+AD2S1200: FAQ
+AD2S1205: FAQ
+AD2S1210: FAQ
+AD2S75: FAQ
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+AD650: FAQ
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+AD654: FAQ
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+AD7134: FAQ
+AD713: FAQ
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AD7547: FAQ
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- AD7982: Driving the reference input
- Connection of exposed pad in AD7982 LFCSP package
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