Texas Instruments | Reduce Effects of Ext RC Filter on Gain Error and Drift in SAR ADC w/Int AFE (Rev. A) | Application notes | Texas Instruments Reduce Effects of Ext RC Filter on Gain Error and Drift in SAR ADC w/Int AFE (Rev. A) Application notes

Texas Instruments Reduce Effects of Ext RC Filter on Gain Error and Drift in SAR ADC w/Int AFE (Rev. A) Application notes
____________________________________________________
Reducing Effects of External RC Filter on Gain Error and
Drift in SAR ADC with Integrated AFE
For applications using SAR ADCs with integrated
analog front ends (AFE), such as theADS8588S, an
external resistor-capacitor (RC) filter at the input may
be necessary to eliminate noise pickup and to protect
the input from electrical overstress. However, this filter
introduces gain error and gain error drift. The purpose
of this TechNote is to show how to calculate the gain
error and gain error drift added by the external RC
filter. Figure 1 shows the integrated AFE in the
ADS8588S and the external balanced RC filter. It is
very important to understand that the external filter
requires a resistor on both the positive and negative
inputs as shown; matching the input impedances is
required for good common mode and noise rejection.
The selection of the external resistor is designed to
limit the input current from an overstress event. A
typical value for this resistance is 10k ohms, and for
maximum protection an external TVS diode is
recommended. The capacitor is selected to set the
filter cutoff frequency. For this example the cutoff
frequency is set to 320Hz for a system monitoring 50
or 60 Hz power signals. The cutoff frequency can be
adjusted according to the design requirements.
Adding the external resistor will affect the room
temperature gain of the data converter as the
resistance is directly in series with the 1M ohm input
impedance of the AFE. Assuming the internal input
impedance and external resistors are known, this gain
error can be mathematically accounted for. However,
the ADS8588S internal impendence has a range of
0.85 Mohm to 1.15 Mohm, which will minimize the
effectiveness of this correction. In this example, the
corrected gain error would be approximately ±0.15%
from internal impedance variation, whereas the
uncorrected gain error would be about 1%±0.15%.
Depending on the magnitude of the external resistor
used, this gain error will differ but for many
applications calibration is used to significantly reduce
this error.
The room temperature gain error added to the system
by the external resistor is given by the equation below.
This gain error equation is based on a voltage divider
relationship and is the additional uncorrected error
from the external resistor. For this example, the
uncorrected gain error due to the external resistor is
0.9901%. For comparison, the ADS8588S internal
maximum gain error is 64 least significant bits which
translates to 0.098% [100*64/(2^16)=0.098%].
GainError(REXT )RoomTemp=
GainError(REXT )RoomTemp=
1
R
1+ IN
REXT
1
= 0.009901 or 0.9901%
1MŸ
1+
10kŸ
A simple two point calibration is typically used to
correct for gain error in a data converter system. This
method will eliminate both the gain error introduced by
the external resistor as well as internal device gain
error. In this calibration method two test signals are
applied and measured at 10% and 90% of the linear
range of the input voltage range. These
measurements are then used to calculate the slope
(m) and offset (b) of the linear transfer function, shown
in the equations below.
Code = PÂVin +b
m=
Code_ max - Code_min
Vmax-Vmin
b = Code_ min - m·Vmin
Vin
REXT
REXT
AIN_xP
RIN
1MŸ
CEXT
PGA
RIN
3rd
2nd
Order
Order
Low-Pass
ADC
Driver
1MŸ
16-B
16-Bit SAR
SAR
CHx_OUT
AIN_xGND
Figure 1. ADS8588S Integrated Analog Front End with external RC filter
SBAA232A – July 2017 – Revised July 2017
Submit Documentation Feedback
Reducing Effects of External RC Filter on Gain Error and Drift in SAR ADC
with Integrated AFE
Copyright © 2017, Texas Instruments Incorporated
1
www.ti.com
Once the calibration coefficients are determined, they
are used to correct for the gain and offset of the data
converter and external components, using the
equation below. Mathematically, this is done by solving
the straight line equation for the input signal. Usually
the calibration method is executed during a factory
automated test, once the coefficients are calculated,
they are stored in memory and used in all subsequent
measurements to minimize the error.
VinCALIBRATE =
Code - b
m
Using calibration can reduce gain and offset errors to
almost negligible levels. After calibration, most of the
residual error is due to noise and drift. Both the
internal 1Mohm resistor and the external filter resistor
will contribute to the system drift. From the ADS8588S
datasheet, the electrical characteristic lists the input
impedance maximum drift at ±25ppm/°C. For this
example a precision low drift external resistor (0.1%
±25ppm/°C) was used. Using a precision low drift
resistor for the filter will achieve best results. To
calculate the effect of the external resistor on drift, first
find the gain error at high temperature. This is done by
calculating the internal and external effective
resistance at high temperature as is shown in the
equations below. For worst case analysis, the signs of
the drift term of the internal and external impedances
must be different. If the signs were to be the same, the
drift errors from the internal and external impedances
would cancel.
RIN(-25ppm¤() =1MQ|€-25ppm/°C·:125°C-25°C; + 1]
RIN(-25ppm¤() = 0.9975 MŸ
REXT(+25ppm¤() =10kQ|€25ppm/°C·:125°C - 25°C;+1]
REXT(+25ppm¤() = 10.025kŸ
Next, to calculate gain error drift, calculate the gain
error at 25⁰C, and at125⁰C.
GainError(REXT)RoomTemp=
1
= 0.009901 or 0.9901%
1MŸ
1+
10kŸ
1
GainError(REXT )125°C =
1+
0Ÿ
NŸ
GainErrorDrift (REXT )=
ûGainError
Â106
ûTemperature
.009950-.009901
Â106 ppm
:125°C-25°C ;
GainErrorDrift (REXT ) = -0.49ppm/°C
In the ADS8588s datasheet the gain error temperature
drift is listed at a maximum ±14ppm/°C when using an
external reference. This is orders of magnitude larger
than the calculated additional drift error introduced by
the external resistor (-0.49ppm/°C), making the
introduced error insignificant. The minimal drift error
introduced by the external components is possible
because both the internal 1Mohm resistance and the
external resistance have low drift. This same
calculation can be done using different external
resistor values. The absolute worst case drift can be
kept relatively low if low drift external resistors (e.g.
±25ppm/⁰C or better) are used. For example, the worst
case drift introduced by a 1Mohm ±25ppm/⁰C external
resistor is about ±12ppm/C..
In summary, an external RC filter is a common way to
reduce noise as well as protect the input circuit. This
circuit will have some impact on the gain error and drift
of the system. This paper demonstrated a method for
calculating gain error and drift. For best accuracy,
calibration can be used. The drift error can be
minimized by using precision low drift external
resistors; this drift will generally be significantly lower
than the device’s drift.
Table 1. Alternative Device Recommendations
Device
Optimized Parameters
Performance Trade-Off
ADS8588S
Input Impedance Max
Drift ±25ppm/°C
Simultaneous Sampling,
16 Bit, 200-kSPS
ADS8688
Input Impedance Max
Drift ±25ppm/°C
MUX, 16 Bit, 500-kSPS
ADS8681
Input Impedance Max
Drift ±25ppm/°C
Single Channel, 16 Bit, 1MSPS
= 0.009950 or 0.995%
Finally, take the difference of the errors over
temperature, divide by the temperature range used
and multiply by one million to convert to ppm.
2
GainErrorDrift (REXT ) =
Table 2. Related Documentation
TIPLADC 3.2
TI Recision Labs ACD 3.2:Understanding
and Calibrating the Offset and Gain for ADC
Systems
TIDUCO9
High-Accuracy Analog Front End Reference
Design Using 16-Bit SAR ADC With ±10-V
Measurement Range
TIDU540A
High-Accuracy AC Voltage and Current
Measurement AFE for Feeder Terminal Unit
Reference Design
TIDU427B
Phase-Compensated, 8-Ch, Multiplexed
Data Acquisition System for Power
Automation Reference Design
Reducing Effects of External RC Filter on Gain Error and Drift in SAR ADC
with Integrated AFE
Copyright © 2017, Texas Instruments Incorporated
SBAA232A – July 2017 – Revised July 2017
Submit Documentation Feedback
IMPORTANT NOTICE FOR TI DESIGN INFORMATION AND RESOURCES
Texas Instruments Incorporated (‘TI”) technical, application or other design advice, services or information, including, but not limited to,
reference designs and materials relating to evaluation modules, (collectively, “TI Resources”) are intended to assist designers who are
developing applications that incorporate TI products; by downloading, accessing or using any particular TI Resource in any way, you
(individually or, if you are acting on behalf of a company, your company) agree to use it solely for this purpose and subject to the terms of
this Notice.
TI’s provision of TI Resources does not expand or otherwise alter TI’s applicable published warranties or warranty disclaimers for TI
products, and no additional obligations or liabilities arise from TI providing such TI Resources. TI reserves the right to make corrections,
enhancements, improvements and other changes to its TI Resources.
You understand and agree that you remain responsible for using your independent analysis, evaluation and judgment in designing your
applications and that you have full and exclusive responsibility to assure the safety of your applications and compliance of your applications
(and of all TI products used in or for your applications) with all applicable regulations, laws and other applicable requirements. You
represent that, with respect to your applications, you have all the necessary expertise to create and implement safeguards that (1)
anticipate dangerous consequences of failures, (2) monitor failures and their consequences, and (3) lessen the likelihood of failures that
might cause harm and take appropriate actions. You agree that prior to using or distributing any applications that include TI products, you
will thoroughly test such applications and the functionality of such TI products as used in such applications. TI has not conducted any
testing other than that specifically described in the published documentation for a particular TI Resource.
You are authorized to use, copy and modify any individual TI Resource only in connection with the development of applications that include
the TI product(s) identified in such TI Resource. NO OTHER LICENSE, EXPRESS OR IMPLIED, BY ESTOPPEL OR OTHERWISE TO
ANY OTHER TI INTELLECTUAL PROPERTY RIGHT, AND NO LICENSE TO ANY TECHNOLOGY OR INTELLECTUAL PROPERTY
RIGHT OF TI OR ANY THIRD PARTY IS GRANTED HEREIN, including but not limited to any patent right, copyright, mask work right, or
other intellectual property right relating to any combination, machine, or process in which TI products or services are used. Information
regarding or referencing third-party products or services does not constitute a license to use such products or services, or a warranty or
endorsement thereof. Use of TI Resources may require a license from a third party under the patents or other intellectual property of the
third party, or a license from TI under the patents or other intellectual property of TI.
TI RESOURCES ARE PROVIDED “AS IS” AND WITH ALL FAULTS. TI DISCLAIMS ALL OTHER WARRANTIES OR
REPRESENTATIONS, EXPRESS OR IMPLIED, REGARDING TI RESOURCES OR USE THEREOF, INCLUDING BUT NOT LIMITED TO
ACCURACY OR COMPLETENESS, TITLE, ANY EPIDEMIC FAILURE WARRANTY AND ANY IMPLIED WARRANTIES OF
MERCHANTABILITY, FITNESS FOR A PARTICULAR PURPOSE, AND NON-INFRINGEMENT OF ANY THIRD PARTY INTELLECTUAL
PROPERTY RIGHTS.
TI SHALL NOT BE LIABLE FOR AND SHALL NOT DEFEND OR INDEMNIFY YOU AGAINST ANY CLAIM, INCLUDING BUT NOT
LIMITED TO ANY INFRINGEMENT CLAIM THAT RELATES TO OR IS BASED ON ANY COMBINATION OF PRODUCTS EVEN IF
DESCRIBED IN TI RESOURCES OR OTHERWISE. IN NO EVENT SHALL TI BE LIABLE FOR ANY ACTUAL, DIRECT, SPECIAL,
COLLATERAL, INDIRECT, PUNITIVE, INCIDENTAL, CONSEQUENTIAL OR EXEMPLARY DAMAGES IN CONNECTION WITH OR
ARISING OUT OF TI RESOURCES OR USE THEREOF, AND REGARDLESS OF WHETHER TI HAS BEEN ADVISED OF THE
POSSIBILITY OF SUCH DAMAGES.
You agree to fully indemnify TI and its representatives against any damages, costs, losses, and/or liabilities arising out of your noncompliance with the terms and provisions of this Notice.
This Notice applies to TI Resources. Additional terms apply to the use and purchase of certain types of materials, TI products and services.
These include; without limitation, TI’s standard terms for semiconductor products http://www.ti.com/sc/docs/stdterms.htm), evaluation
modules, and samples (http://www.ti.com/sc/docs/sampterms.htm).
Mailing Address: Texas Instruments, Post Office Box 655303, Dallas, Texas 75265
Copyright © 2017, Texas Instruments Incorporated
Was this manual useful for you? yes no
Thank you for your participation!

* Your assessment is very important for improving the work of artificial intelligence, which forms the content of this project

Related manuals

Download PDF

advertising