CS5101A datasheet(24/39 Pages) CIRRUS

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CS5101A CS5102A

DS45F6

capacitor exhibits an impedance of less than 2

Ω at

frequencies greater than 16 kHz. A high-quality

tantalum capacitor in parallel with a smaller ceram-

ic capacitor is recommended.

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 11 can be used to help

calculate the optimum value of R for a particular

reference. The term “fpeak” is the frequency of the

peak in the output impedance of the reference be-

fore the resistor is added.

Figure 11. Reference Connections

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 recommended refer-

ence voltage is 4.5 volts. The CS5101A and

CS5102A can actually accept reference voltages up

to the positive analog supply. However, the buffer's

offset may increase as the reference voltage ap-

proaches VA+ thereby increasing external drive re-

quirements at VREF. A 4.5V reference is the

maximum reference voltage recommended. This al-

lows 0.5V headroom for the internal reference buff-

er. Also, the buffer enlists the aid of an external

0.1

µF ceramic capacitor which must be tied be-

tween its output, REFBUF, and the negative analog

supply, VA-. For more information on references,

consult application note

AN004, Voltage References for CS5012A / CS5014 /CS5016 /

CS5101A / CS5102A / CS5126 Series of A/D Converters.

6.2

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 speci-

fied accuracy. Figure 12 shows this operation. Dur-

ing coarse-charge the charge on the capacitor

array first settles to the buffered version of the an-

alog input. This voltage may be offset from the ac-

tual input voltage. During fine-charge, the charge

then settles to the accurate unbuffered version.

Fine-charge settling is specified as a maximum of

1.125

µs (CS5101A) or 5.625 µs (CS5102A) for an

analog source impedance of less than 50

Ω. In ad-

dition, the comparator requires a source imped-

ance of less than 400

Ω around 2 MHz for stability.

The source impedance can be effectively reduced

at high frequencies by adding capacitance from

AIN to ground (typically 200 pF). However, high

DC source resistances will increase the input's RC

time constant and extend the necessary acquisi-

tion time. For more information on input amplifiers,

consult the application note,

AN006, Buffer Amplifiers for CS5012A / 14 / 16 / CS5101A /

CS5102A / CS5126 Series of A/D Converters.

Figure 12. Charge Settling Time

6.3

Sleep Mode Operation

The CS5101A and CS5102A include a SLEEP pin.

When SLEEP is active (low) each device will dissi-

pate very low power to retain its calibration memo-

VR E F

RE F B UF

VA-

0.1

µF

-5V

0.01

µF

+V ee

C S 5101A

C S 5102A

ref

π C1 C2

() fpeak

⋅⋅

()

----------------------------------------------------------------

A cquisition Tim e (us)

+ 200

-1 0 0

-4 0 0

+ 100

-2 0 0

-3 0 0

Fine -C harge

C oarse -C harge

0.25

0 .5

0 .7 5

1 .0

1.0

2.0

3 .0

4.0

8 M H z C lo ck

2.0 M H z C lock

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