Cirrus-logic CS5460A User Manual download pdf (Page 27)

CS5460A

DS487F5 27

that the Energy Register is read at least once in ev-

ery 5 second span.

3.5 Oscillator Characteristics

XIN and XOUT are the input and output, respec-

tively, of an inverting amplifier to provide oscillation

and can be configured as an on-chip oscillator, as

shown in Figure 15. The oscillator circuit is de-

signed to work with a quartz crystal or a ceramic

resonator. To reduce circuit cost, two load capaci-

tors C1 are integrated in the device, one between

XIN and DGND, one between XOUT and DGND.

Lead lengths should be minimized to reduce stray

capacitance.

To drive the device from an external clock source,

XOUT should be left unconnected while XIN is

driven by the external circuitry. There is an amplifi-

er between XIN and the digital section which pro-

vides CMOS level signals. This amplifier works

with sinusoidal inputs so there are no problems

with slow edge times.

The CS5460A can be driven by a clock ranging

from 2.5 to 20 MHz. The K divider must be set to

the appropriate value such that MCLK/K will be be-

tween 2.5 MHz and 5 MHz. The K divider value is

set with the K[3:0] bits in the Configuration Regis-

ter. As an example, if XIN = MCLK = 15 MHz, and

K is set to 5, then MCLK/K = 3 MHz, which is a val-

id value for MCLK/K. Note that if the K[3:0] bits are

all set to zero, the value of the K divider value is 16.

3.6 Analog Inputs

The CS5460A accommodates a full-scale differen-

tial input voltage range of

±250 mV on both input

channels. (If the PGA setting on the current chan-

nel is set for the 50x gain setting instead of the 10x

gain setting, then the differential full-scale input

range on the current channel reduces to

±50 mV.)

System calibration can be used to increase or de-

crease the full scale span of the converter as long

as the calibration register values stay within the

limits specified. See Section 3.8, Calibration, for

more details.

3.7 Voltage Reference

The CS5460A is specified for operation with a

+2.5 V reference between the VREFIN and VA-

pins. A reference voltage must be supplied to the

VREFIN pin for proper operation of the two ADCs.

The CS5460A includes an internal 2.5 V reference,

available on the VREFOUT pin, that can be used

as the reference input voltage by connecting the

VREFOUT pin to the VREFIN pin. If lower temper-

ature drift is desired, an external reference can be

used; in which case the VREFOUT pin should be

left unconnected.

3.8 Calibration

3.8.1 Overview of Calibration Process

The CS5460A offers digital calibration. Each cali-

bration sequence will be executed by setting/clear-

ing one or more of the 8 bits in the calibration

command word. For both channels, there are cal-

ibration sequences for both AC and DC purposes.

Regardless of whether an AC or DC calibration se-

quence is desired, there are two basic types of cal-

ibrations: system offset and system gain. During

the calibration sequences, proper input calibra-

tion signals to the “+” and “-” pins of the volt-

age-/current-channel inputs must be supplied.

These input calibration signals represent

full-scale levels (for gain calibrations) and ground

input levels (for offset calibrations).

The AC and DC calibration sequences are differ-

ent. Depending on the specific metering applica-

tion and accuracy requirements, some or all of the

calibration sequences may not be executed. (This

Oscillator

Circuit

DGND

XIN

XOUT

C1 =

22 pF

C2 =

Figure 15. Oscillator Connection

1 2 ... 22 23 24 25 26 27 28 29 30 31 32 ... 53 54

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