CS5372 datasheet(12/22 Pages) CIRRUS

12 / 22 page

CS5371/CS5372

DS255PP2

VREF- pin connected to VA-. Because most 2.5 V

voltage references require a power supply voltage

greater than 3 V to operate, when powering the

voltage reference from dual ±2.5 V or ±3.0 V sup-

plies the reference voltage into the VREF+ pin is

defined relative to the VA- supply.

The selected voltage reference should produce less

than 1

µVrms of noise in the measurement band-

width on the VREF+ pin. The CS5376 digital filter

output word rate selection determines the band-

width over which voltage reference noise affects

the CS5371/CS5372 modulator dynamic range.

6.1.

Voltage Reference Configurations

For a 2.5 V reference, the Linear Technology

LT1019-2.5 voltage reference yields low enough

noise if the output is filtered with a low pass RC fil-

ter as shown in Figure 6. The filtered version in

Figure 6 is acceptable for most spectral measure-

ment applications, but a buffered version with low-

er source impedance, Figure 7, may be preferred

for DC measurement applications. The configura-

tion shown in Figure 7 can use a Linear Technolo-

gy LT1077 or similar low voltage op-amp to buffer

the voltage reference output.

6.2.

VREF Input Impedance

Due to the dynamic switched-capacitor input archi-

tecture the input current required from the voltage

reference, and thus the input impedance of the

modulator VREF+ pin, will change any time

MCLK is changed. The input impedance of the

voltage reference input is calculated similar to the

analog signal input impedance as [1 / (f * C)]

where f is the modulator clock frequency, MCLK,

200

Ω

To VREF+

Option A

To VREF -

0.1

µF

+3 V

2.5 REF

0.1

µF

-3 V

0.1

µF

Figure 6. 2.5 Voltage Reference Option A

49.9

Ω

100

Ω

10k

Ω

100

µF

To VREF+

Option B

OPAMP

To VREF -

0.1

µF

Tant

100

µF

+3 V

2.5 REF

0.1

µF

µu

-3 V

0.1

µF

µu

Figure 7. 2.5 Voltage Reference Option B

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