CS5101A CS5102A
+200
+100
0
+Vee
20 VREF
V
ref
21 REFBUF
10
µF
0.01 µF
-100
-200
Coarse-Charge
Fine-Charge
0.1µF
23
CS5101A
OR
VA-
R*
-300
-400
CS5102A
-5V
0.5
2.0
0.75
3.0
1.0
4.0
8 MHz Clock 0.25
2.0 MHz Clock 1.0
1
R =
Acquisition Time (us)
2π (C + C ) f
peak
1
2
Figure 9. Reference Connections
Figure 10. Charge Settling Time
(8 and 2.0 MHz Clocks)
ence must supply a maximum load current of
5 µA peak-to-peak (0.5 µA typical). An output
impedance of 2 Ω will therefore yield a maxi-
mum error of 10.0 µV. With a 4.5 V reference and
LSB size of 138 µV this would insure approxi-
mately 1/14 LSB accuracy. A 10 µF capacitor
exhibits an impedance of less than 2 Ω at fre-
quencies greater than 16 kHz. A high-quality
tantalum capacitor in parallel with a smaller ce-
ramic capacitor is recommended.
reference voltage approaches VA+ thereby in-
creasing external drive requirements at VREF. A
4.5V reference is the maximum reference voltage
recommended. This allows 0.5V headroom for
the internal reference buffer. Also, the buffer en-
lists the aid of an external 0.1 µF ceramic
capacitor which must be tied between its output,
REFBUF, and the negative analog supply, VA-.
For more information on references, consult "Ap-
plication Note: Voltage References for the
CS501X Series of A/D Converters".
Peaking in the reference’s output impedance can
occur because of capacitive loading at its output.
Any peaking that might occur can be reduced by
placing a small resistor in series with the capaci-
tors. The equation in Figure 9 can be used to help
calculate the optimum value of R for a particular
Analog Input Connection
The analog input terminal functions similarly to
the VREF input after each conversion when
switching into the track mode. During the first
six master clock cycles in the track mode, the
buffered version of the analog input is used for
coarse-charging the capacitor array. An additional
period is required for fine-charging directly from
AIN to obtain the specified accuracy. Figure 10
shows this operation. During coarse-charge the
charge on the capacitor array first settles to the
buffered version of the analog input. This voltage
may be offset from the actual input voltage. Dur-
ing fine-charge, the charge then settles to the
accurate unbuffered version.
reference. The term "f " is the frequency of
peak
the peak in the output impedance of the reference
before the resistor is added.
The CS5101A and CS5102A can operate with a
wide range of reference voltages, but signal-to-
noise performance is maximized by using as
wide a signal range as possible. The recom-
mended reference voltage is 4.5 volts. The
CS5101A and CS5102A can actually accept ref-
erence voltages up to the positive analog supply.
However, the buffer’s offset may increase as the
DS45F2
21
CIRRUS [ CIRRUS LOGIC ]
