a AD7714/AD7715 Instrumentation Converter FAQs: Calibration

a  AD7714/AD7715 Instrumentation Converter FAQs: Calibration
a
AD7714/AD7715 Instrumentation Converter FAQs:
Calibration
How is self-calibration implemented on the AD7714/AD7715?
For calibration to occur regardless of which calibration mode is used on the selected channel, the ADC’s on-chip
microcontroller must record the modulator output for two different analog input conditions. These are the “zero-scale” and
“full-scale” points. With these conversions, the microcontroller can calculate the gain slope for the input to output transfer
function of the converter. Internally, the part works with a resolution of 33 bits to determine its conversion result of either 16
bits or 24 bits.
In self-calibration mode, the ADC determines the calibration points internal to the ADC. The zero-scale point used to
determine the calibration coefficients is with both inputs shorted (i.e., AIN(+) = AIN(–) = Vref) internally within the ADC.
Signals connected to the analog input pins of the device will not affect the calibration procedure as long as they are within the
common range of the input. The full-scale coefficient is determined by applying a voltage of VREF to the modulator inputs.
Self-calibration mode is invoked in the AD7714 by writing to the mode bits (MD2, MD1 and MD0 bits) in the Mode register.
With the AD7715, bits MD1 and MD0 in the Setup register are used to initiate a calibration.
In this calibration mode, the shorted inputs node is switched in to the modulator first and a conversion is performed. The
VREF node is then switched in and another conversion is performed. DRDY can be used to determine when the calibration is
complete and new data pertaining to the analog input is available at the output. DRDY goes high on the initiation of the
calibration and will not return low until a conversion result on the external analog input is available. The self-calibration
procedure takes into account the selected gain on the PGA.
What is system calibration and how is it implemented?
System calibration allows the converter to compensate for external system gain and offset errors as well as its own internal
errors. Calibration is basically a conversion process on two specific input voltages (zero scale and full-scale) from which the
offset error coefficient and full-scale error coefficient are determined. With system calibration, the zero scale voltage and
full-scale voltage must be applied to the ADC by the user.
System calibration is a two-step process. The zero-scale point must be presented to the converter first. This voltage is applied
to the analog input of the converter before the zero-scale system calibration step is initiated and must remain stable until the
step is complete. System calibration is initiated by writing the appropriate values to the MD2, MD1 and MD0 bits of the
mode register when using the AD7714 or bits MD1 and MD0 in the setup register on the AD7715. The DRDY output from
each device indicates when the step is complete by going low or the mode bits can be monitored via software - these return to
normal mode when calibration is complete. After the zero-scale point is calibrated, the full-scale point is applied and the fullscale system calibration process is initiated by again writing the appropriate code to the MD bits. The full-scale voltage must
be set up before the calibration is initiated and it must remain stable throughout the duration of the calibration. DRDY goes
low at the end of this second step to indicate that the system calibration is complete.
The calibration procedure is dependent on whether unipolar mode or bipolar mode is used. In the unipolar mode, the system
calibration is performed between the two endpoints of the transfer function while in the bipolar mode, it is performed
between mid-scale and positive full-scale.
When performing a system calibration, the zero-scale voltage and full-scale voltage must be switched into the analog input
channel of the ADC. This can be performed by using a low Ron SPDT (Single Pole Double Throw) CMOS switch. One of
the switch inputs can be connected to the analogue input which represents the full-scale value while the other input can be
connected to the zero-scale voltage. Using this switch ensures that the signal chain on both analog inputs for the zero-scale
calibration and full-scale calibration is identical. By so doing, the system zero-scale calibration will compensate for the
insertion loss of the switch. The ADG736 is a dual SPDT switch with a Ron of < 4Ohms and matching of better than
0.4Ohms.
What are the advantages and disadvantages associated with background calibration when using the
AD7714?
Background calibration mode is an additional feature present in the AD7714. In background calibration mode, the ADC
interleaves its calibration procedure with its normal conversion sequence. Background calibration mode is basically the same
as self-calibration but the calibrations and data conversions are interleaved. The AD7714 interleaves zero-scale calibrations
with conversion results. There are no full-scale calibrations performed. Any drift errors associated with these converters
appear as offset errors. The gain drift with temperature is typically 0.2ppm/ °C.
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AD7714/AD7715 Instrumentation Converter FAQs:
Calibration
When invoked, the background calibration mode reduces the output data rate of the converter by a factor of six but the –3 dB
bandwidth remains unchanged. Reduced output data rate is the only disadvantage associated with this mode of operation.
The key advantage is that the part is continually performing calibrations and automatically updating its zero-scale calibration
coefficient. As a result, the effects of temperature drift, supply sensitivity and time drift are automatically removed.
It is important to note that a full-scale calibration (system or self) must be performed before the AD7714 is put into
background calibration mode as no full-scale calibrations are implemented in this mode.
When should a calibration be performed on the AD7714/AD7715 family of sigma delta ADCs?
A calibration must be performed when there is a change in
• gain
• Update Rate
• Temperature
or when Switching between channels that share coefficients registers but have different operating conditions.
With any gain change, there will be a matching error between gain ranges and this error needs to be calibrated out. In
switching between gains 1,2,4 and 8, it may be possible to switch without the need for a calibration as this gain is
implemented by multiple sampling of the input capacitor. This is accurately controlled and minimal error is introduced. In
switching between any other of the gain ranges, a calibration is required as the gain is implemented by scaling of capacitors.
Capacitor errors exist due to processing errors etc.
If the update rate is changed for any reason, a calibration is required to alter the calibration coefficients in order to get
accurate data results. The ADC architecture uses gross scaling factors internally when determining the calibration
coefficients. Thus, the coefficients used by the part vary substantially across the range of update rates available.
Drift errors are due to changes in temperature. Calibration can be used effectively to remove any errors associated with
temperature drift. Self-calibration will remove the effects of temperature drift within the ADC itself. System calibration can
be used to remove the drift errors in the ADC itself and also the drift errors associated with the front-end signal conditioning
circuitry.
When pseudo-differential input mode is used on the AD7714, certain input channels share calibration register pairs.
Therefore, if two channels share a calibration register pair, it is important that a calibration be performed when switching
between these channels if the operating conditions (gain or update rate) between the two differ. If the two channels are
configured identically and self-calibration is used, there is no need to perform a calibration when switching channels.
However, if system calibration is being used, a calibration should be performed when switching between channels as the
applied analog input levels may be different.
How often should a calibration be implemented in a system?
To determine the frequency of calibrations within a system, consider.
• What accuracy is required from the converter?
• How does drift performance of the ADC limit performance?
• Over what temperature range does the system operate?
Taking these three questions into account will give some indication as to how often a calibration is required. Other system
parameters should also be considered when determining how often to calibrate. These are all related to circuit sensitivities to
temperature change as follows:
• Parasitic thermocouple effects.
• Gain drift due to the reference temperature coefficient.
• Drift sources external to the converter.
In general, the higher the accuracy requirement, the more often a calibration will be required in order to maintain system
accuracy. After a calibration has been performed, high-resolution converters like the AD7714 will have some offset and gain
drift associated with them. The AD7714’s offset drift due to temperature is typically 0.3uV/°C and the gain drift is typically
0.5ppm of full-scale range/°C. In a system with large temperature variation, using background calibration will continuously
remove the effects of temperature drift within the ADC as zero-scale calibrations are interleaved with normal conversions. In
accessing the complete effects on accuracy due to temperature, the temperature effects due to parasitic thermocouples and
drift sources external to the converter also need to be factored into the equation.
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AD7714/AD7715 Instrumentation Converter FAQs:
Calibration
If factory system calibration is performed, can temperature drift errors associated with the ADC be
removed in the field without performing additional system calibrations?
System calibration can be easily implemented as part of a factory calibration but it is much harder to implement in the field
since the system zero-scale and system full-scale voltages must be applied to the analog inputs during the calibration. These
input voltages are not always easily available when operating in the field. Following the factory system calibration, the user
still has the issue of removing ADC drift errors in the field due to temperature changes. The following outlines a method of
overcoming this problem. This is outlined in two sections below: firstly the system factory calibration and secondly the field
calibration.
Factory-Calibration:
•
•
•
•
Perform a Self-Calibration at the required operating gain and update rate.
Read and store the calibration register contents. Offset=Z0 and Gain=G0.
Perform a System Calibration at the required operating gain and update rate as before.
Read and store the calibration register contents. Offset =ZS and Gain=GS.The system can be shipped to the field with
the system calibration coefficients loaded to the ADC. If the ambient temperature changes, the offset and gain drift
errors need to be calibrated out of the system as follows:
Field-Calibration:
•
•
•
Perform a Self-Calibration at the required operating gain and update rate. The Gain and update rate must remain the
same as in the original self and system calibrations.
Read the calibration register contents. Offset=Z1 and Gain=G1.
Calculate the new calibration coefficients.
ZN=ZS+(Z1-Z0)
GN=GS*(G1/G0)
• Write ZN and GN to the calibration registers.
This procedure retains the original system calibration but adjusts the coefficients to remove errors due to temperature drift in
the ADC. This procedure removes drift errors due to the ADC only. Drift errors due to the analog front-end signal chain are
not removed.
Can the calibration coefficients be altered manually to cater for input ranges other than the nominal
ranges?
This situation arises when the user has a specific input range other than the nominal range but cannot implement a system
calibration as the zero and full-scale voltages are not available during calibration. The following description shows how to
alter the coefficients to accommodate input ranges other than 0 to Vref and +/-Vref. The part must first be calibrated using
the self-calibration procedure with the appropriate gain, input range, update rate and bipolar/unipolar input range selected.
The coefficients generated from the self-calibration procedure are used to generate new coefficients.
For example, if the required input voltage Vin is represented by:
Vin = A * VREF + B
Where B is an offset voltage and A * Vref is the input span.
The self-calibration procedure operates on shorted inputs for zero scale calibration and VREF for full-scale calibration which
results in A = 1 and B = 0. The following procedure can be implemented so as to accommodate input ranges other than the
nominal 0 to Vref or +/-Vref. The offset B is subtracted so that an output code of zero will be obtained when the analog input
voltage is B. The span is adjusted to A * VREF so that this full-scale voltage will output a full-scale code.
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AD7714/AD7715 Instrumentation Converter FAQs:
Calibration
• Perform a self-calibration and read back the calibration coefficients and let:
• Z0=Zero-scale coefficient and
• F0=Full-scale coefficient.
These coefficients can then be modified to cater for the new input range using the following formulae to generate the new
coefficients ZN and FN.
ZN= Z0 + (B * 2^20 / (SPAN * F0 / 2^24))
FN= F0 / A
Where SPAN is the full-scale voltage span under nominal conditions which equals VREF/Gain in unipolar mode and
2VREF/gain in bipolar mode.
B is the offset voltage in volts and A is the scaling factor applied to the nominal span. A must have a value between 0.8 and
1.05 for guaranteed operation.
• Write ZN and FN to the ADC calibration registers to accommodate the new input range.
For example, suppose the ADC has a zero-scale coefficient of 2,165,373 and a full-scale coefficient of 5,416,211 following a
self-calibration in unipolar mode. Since unipolar mode is used, the range used for the calibration is 0 to VREF where VREF
is equal to 2.5V when a 5 V power supply is used. If the user requires an analog input range of 0.2 V to 2.6 V then, B is
equal to 0.2 while A equals (2.6 – 0.2) / 2.5 = 0.96. The new zero-scale coefficient ZN is
ZN = 2,165,373 + (0.2 * 2^20 / (2.5 * 5,416,211 / 2^24)) = 2,425,218 and
FN = 5,416,211 / 0.96 = 5,641,886.
This scheme is useful only if the user knows the exact upper and lower limits of the desired input range and the ratio of the
actual input span to the nominal input span.
With this method, the ADC will continue to meet the noise specifications given in the datasheet if the user ensures that the
variable A is between 0.8 and 1.05. For example, the AD7714 has an rms noise spec of 1.5 uV in buffered mode when
operated with a 5V power supply, a 10 Hz update rate and a gain of 1 which results in a signal range of 0 to 2.5 V in unipolar
mode. If the input range is altered so that it varies from 0.2V to 2.5V as in the above example, the noise will continue to be
1.5 uV if the operating conditions (update rate, gain, etc) are not changed. With the original range, the peak to peak
resolution equals log (2.5V / 6.6 * 1.5uV) / log 2 = 18 bits when rounded to the nearest 0.5 bit. With the altered range, the
peak to peak resolution equals log (2.4V / 6.6 * 1.5uV) / log 2 = 18 also when rounded to the nearest 0.5 bit.
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