Q
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?
A
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.