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ADP1046AW Datasheet(PDF) 34 Page - Analog Devices

Part No. ADP1046AW
Description  Digital Controller for Isolated
Download  88 Pages
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Maker  AD [Analog Devices]
Homepage  http://www.analog.com

ADP1046AW Datasheet(HTML) 34 Page - Analog Devices

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Data Sheet
Rev. 0 | Page 34 of 88
The voltage sense inputs are optimized for sensing signals at
1 V (the usable input range is 1.4 V). In a 12 V system, a 12:1
resistor divider is required to reduce the 12 V signal to below
1.4 V. It is recommended that the output voltage of the power
supply be reduced to 1 V at this pin for best performance. The
tolerance of the resistor divider introduces errors that need to be
trimmed. The ADP1046AW has enough trim range to trim out
errors introduced by resistors with a tolerance of 0.5% or better.
The VS1, VS2, and VS3 ADCs produce a digital code equal to
VSx/1.6 × 4096. The ADCs output a digital word of 2560
decimal (0xA00) in Bits[15:4] of Register 0x15, Register 0x16,
and Register 0x17 when there is exactly 1 V at their inputs.
The VS3± inputs require a gain trim. Set the output regulation
point to 100% of the nominal value (Register 0x31 = 0xA0).
Enable the power supply with no-load current. The power
supply output voltage is divided down by the VS3 resistor
divider to give 1 V across the VS3+ and VS3− differential input
pins. The VS3 trim register (Register 0x3A) is adjusted until
the output voltage is at the desired value. This step should be
performed before any other trim routine. The VS3 voltage
value in Register 0x17[15:4] reads 2560 decimal (0xA00).
The VS1 input requires a gain trim. Enable the power supply
with no-load current. It is recommended that the VS1 voltage
be divided down by the VS1 resistor divider to give 1 V at the
VS1 pin. The VS1 trim register (Register 0x38) is adjusted until
the VS1 value in Register 0x15[15:4] reads 2560 decimal (0xA00).
The VS2 input requires a gain trim. Enable the power supply
with no-load current. It is recommended that the VS2 voltage
be divided down by the VS2 resistor divider to give 1 V at the
VS2 pin. The VS2 trim register (Register 0x39) is adjusted until
the VS2 value in Register 0x16[15:4] reads 2560 decimal (0xA00).
The RTD input requires two trims: one for the current source
and one for the ADC. To use the internal linearization scheme,
additional trimming procedures are required.
Trimming the Current Source
Bits[7:6] of Register 0x11 set the value of the current source to
10 µA, 20 µA, 30 µA, or 40 µA. Bits[5:0] of Register 0x11 can be
used to fine-tune the current value. By fine-tuning the internal
current source, component tolerance can be compensated for
and errors can be minimized. One LSB in Bits[5:0] = 160 nA.
A decimal value of 1 adds 160 nA to the current source set by
Bits[7:6]; a decimal value of 63 adds 63 × 160 nA = 10.08 µA
to the current source set by Bits[7:6].
To program a value for the current source, select the nearest
possible option (10 µA, 20 µA, 30 µA, or 40 µA) using
Register 0x11[7:6]. Then use Register 0x11[5:0] to achieve
the finer step size.
For example, to use a value of 46 µA as the current source,
follow these steps:
1. Place a known resistor (Rx) from RTD to AGND.
2. Set Register 0x11[7:6] to 11 (40 µA).
3. Increase the value of Register 0x11[5:0] one LSB at a time
until the voltage at the RTD pin is VRTD = 46 µA × Rx.
The current source is now calibrated and is set to the factory
default value.
Trimming the ADC
Due to the nonlinear nature of the thermistor, two trimming
options can be used.
Using the Internal Linearization Scheme
The first option uses the internal linearization scheme with
46 µA RTD current, which provides an accurate reading in °C
read in Register 0x1B in decimal format.
A 100 kΩ, 1% NTC thermistor with beta = 4250, 1% (such as
the NCP15WF104F03RC) in parallel with an external resistor
of 16.5 kΩ, 1%, should be used with the ADP1046AW. With this
NTC thermistor and resistor combination, the ADP1046AW
default current source trim is set to 46 µA to achieve the best
possible accuracy over temperatures ranging from 85°C to 125°C.
If an external microcontroller is used, the RTD ADC code in
Register 0x1A can be fed into the microcontroller and a different
linearization scheme can be implemented in terms of a best-fit
polynomial for the selected NTC characteristics.

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