Cirrus Logic AN366 - Manual (Page 9)

AN366
AN366REV2 9
4.1.2.2 Non-full-scale Gain Calibration
When resources are limited, it may be necessary to provide non-full-scale amplitudes and perform built-in
calibration to provide the maximum voltage and current during calibration. To perform a non-full-scale cali-
bration, the initial gain register conditions of the device must be identified before calibration. Usually, initial
gain register conditions are set to a default value of one, but this is not required. Instead, the initial gain reg-
ister conditions are set to accommodate the non-full-scale input calibration. Before calibration is executed,
the gain register can be set using the following equations:
where:
V
GAIN(pre)
Value stored in voltage gain register (page 16, address 35) before calibration starts
I
GAIN(pre)
Value stored in current gain register (page 16, address 33) before calibration starts
V
MAX
Maximum voltage of the meter defined by customer
I
MAX
Maximum current of the meter defined by customer
V
REF
Voltage of the line just before calibration as measured with reference meter assumes
stable input
I
REF
Load current just before calibration as measured with reference meter assumes stable
input
Follow the steps below to perform a non-full-scale gain calibration:
1. Set the line voltage and load current V
REF
and I
REF
, respectively.
2. Confirm that the reference meter shows V
REF
and I
REF
of the input.
3. Set V
GAIN(pre)
per Equation 6 and I
GAIN(pre)
per Equation 7.
4. Send the calibration command.
5. After calibration, the meter is adjusted for a full-scale voltage of V
MAX
and I
MAX
and will currently be
measuring the V
REF
and I
REF
measurements.
Reference Limits
The calibration line voltage (V
REF
) or load current (I
REF
) must not be set too low. It is recommended to keep
the register values at a minimum of ½ of the maximum levels. Since the gain register can be set to a maxi-
mum value of 4, the input could be set to ¼ of the maximum levels. It is not recommended to set the input
to ¼ of the maximum levels due to variations in setup conditions. If the input is too low, the gain register will
set the default value of one after calibration.
V
GAIN pre  
V
MAX
V
REF
--------------- 2
22
 = [Eq: 6]
I
GAIN pre  
I
MAX
I
REF
------------ 2
22  = [Eq: 7]