Cirrus-logic CS5460A User Manual download pdf (Page 29)

CS5460A

DS487F5 29

Note that when the calibration command is sent to

the CS5460A, the device must not be performing

A/D conversions (in either of the two acquisitions

modes). If the CS5460A is running A/D conver-

sions/computations in the ‘continuous computation

cycles’ acquisition mode (C = 1), the Pow-

er-Up/Halt Command must be issued first to termi-

nate A/D conversions/computations. If the

CS5460A is running A/D conversions/computa-

tions in the ‘single computation cycle’ data acquisi-

tion mode (C = 0), the Power-Up/Halt Command

must be issued first (unless the computation cycle

has completed) before executing any calibration

sequence. The calibration sequences will not run

if the CS5460A is running in either of the two avail-

able acquisition modes.

3.8.4 Calibration Signal Input Level

For both the voltage and current channels, the dif-

ferential voltage levels of the calibration signals

must be within the specified voltage input limits (re-

fer to “Differential Input Voltage Range” in Section

1., Characteristics & Specifications). For the volt-

age channel the peak differential voltage level can

never be more than

500 mV

P-P

. The same is true

for the current channel if the current channel input

PGA is set for 10x gain. If the the current channel’s

PGA gain is set to 50x, then the current channel’s

input limits are

100 mV

P-P

Note that for the AC/DC gain calibrations, there is

an absolute limit on the RMS/DC voltage levels (re-

spectively) that are selected for the voltage/current

channel gain calibration input signals. The maxi-

mum value that the gain register can attain is 4.

Therefore, for either channel, if the voltage level of

a gain calibration input signal is low enough that it

causes the CS5460A to attempt to set either gain

will be invalid, and after this occurs, all CS5460A

results obtained when the part is running A/D con-

versions will be invalid.

3.8.5 Calibration Signal Frequency

The frequency of the calibration signals must be

less than 1 kHz (assume MCLK/K = 4.096 MHz

and K = 1). Optimally, the frequency of the calibra-

tion signal will be at the same frequency as the fun-

damental power line frequency of the power

system that is to be metered.

3.8.6 Input Configurations for Calibrations

Figure 16 shows the basic setup for gain calibra-

tion. If a DC gain calibration is desired, a positive

DC voltage level must be applied, such that it truly

represents the absolute maximum peak instanta-

neous voltage level that needs to be measured

across the inputs (including the maximum

over-range level that must be accurately mea-

sured). In other words, the input signal must be a

positive DC voltage level that represents the de-

sired absolute peak full-scale value. However, in

many practical power metering situations, an AC

signal is preferred over a DC signal to calibrate the

gain. To perform AC gain calibration instead of DC,

an AC reference signal should be applied that is

set to the desired maximum RMS level. Because

the voltage/current waveforms that must be mea-

sured in most power systems are approximately si-

nusoidal in nature, the RMS levels of the AC gain

calibration input signals may need to be set signif-

icantly lower than the voltage/current channel’s

maximum DC voltage input level. This must be

done in order to avoid the possibility that the peak

values of the AC waveforms that are to be mea-

sured will not register a value that would be outside

the available output code range of the voltage/cur-

rent A/D converters. For example, on the voltage

channel, if the Voltage Channel Gain Register is

set to it’s default power-on value of 1.000... before

calibration, then the largest pure sinusoidal wave-

form that can be used in AC calibration is one

whose RMS-value is ~0.7071 of the value of the

voltage channel’s peak DC input voltage value of

500 mV

P-P

. Thus the maximum value of the input

sinusoid would be ~176.78 mV (rms). But in many

practical power metering situations, the RMS volt-

age input level of the AC gain calibration signal

may be reduced even further, to allow for some

over-ranging capability. A typical sinusoidal cali-

bration value which allows for reasonable

over-range margin would be 0.6 of the voltage/cur-

rent channel’s maximum input voltage level. For

the voltage channel, such a sine-wave would have

a value of 0.6 x 250 mV

rms

=150mV

rms

For the offset calibrations, there is no difference

between the AC and DC calibration signals that

must be supplied: simply connect the “+” and “-’

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Cirrus-logic CS5460A User Manual Page 29

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