CS5531-AS datasheet(26/52 Pages) CIRRUS

26 / 52 page

CS5531/32/33/34

DS289PP5

The RV bit is cleared after the configuration regis-

ter is read. The on-chip registers are initialized to

the following default states:

After reset, the RS bit should be written back to

logic 0 to complete the reset cycle. The ADC will

return to the command mode where it waits for a

valid command. Also, the RS bit is the only bit in

the configuration register that can be set when ini-

tiating a reset (i.e. a second write command is need-

ed to set other bits in the Configuration Register

after the RS bit has been cleared).

2.3.3. Input Short

The input short bit allows the user to internally

ground all the inputs of the multiplexer. This is a

useful function because it allows the user to easily

test the grounded input performance of the ADC

and eliminate the noise effects due to the external

system components.

2.3.4. Guard Signal

The guard signal bit is a bit that modifies the func-

tion of A0. When set, this bit outputs the common

mode voltage of the instrumentation amplifier on

A0. This feature is useful when the user wants to

connect an external shield to the common mode po-

tential of the instrumentation amplifier to protect

against leakage. Figure 8 illustrates a typical con-

nection diagram for the guard signal.

2.3.5. Voltage Reference Select

The voltage reference select (VRS) bit selects the

size of the sampling capacitor used to sample the

voltage reference. The bit should be set based upon

the magnitude of the reference voltage to achieve

optimal performance. Figures 9 and 10 model the

effects on the reference’s input impedance and in-

put current for each VRS setting. As the models

show, the reference includes a coarse/fine charge

buffer which reduces the dynamic current demand

of the external reference.

The reference’s input buffer is designed to accom-

modate rail-to-rail (common-mode plus signal) in-

put voltages. The differential voltage between the

VREF+ and VREF- can be any voltage from 1.0 V

up to the analog supply (depending on how VRS is

configured), however, the VREF+ cannot go above

VA+ and the VREF- pin can not go below VA-.

Note that the power supplies to the chip should be

established before the reference voltage.

2.3.6. Output Latch Pins

The A1-A0 pins of the ADCs mimic the D21-

D20/D5-D4 bits of the channel-setup registers if

the output latch select (OLS) bit is logic 0 (default).

If the OLS bit is logic 1, A1-A0 mimic the output

latch bit settings in the configuration register.

These two options give the user a choice of allow-

ing the latch outputs to change anytime a different

CSR is selected for a conversion, or to allow the

latch bits to remain latched to a fixed state (deter-

mined by the configuration register bit) for all CSR

selections. In either case, A1-A0 can be used to

control external multiplexers and other logic func-

tions outside the converter. The A1-A0 outputs can

sink or source at least 1 mA, but it is recommended

Configuration Register:

00000000(H)

Offset Registers:

00000000(H)

Gain Registers:

01000000(H)

Channel Setup Registers: 00000000(H)

C o m m on M ode = 2 .5 V

out m

center

out p

+5 V A +

V+

V-

C S 5 5 3 1 /32/33 /3 4

AIN+

AIN -

A0 /G UARD

Figure 8. Guard Signal Shielding Scheme

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