Mailing Address: PO Box 11400, Tucson, AZ 85734 鈥?Street Address: 6730 S. Tucson Blvd., Tucson, AZ 85706 鈥?Tel: (520) 746-1111
Telex: 066-6491 鈥?FAX (520) 889-1510 鈥?Product Info: (800) 548-6132 鈥?Internet: www.burr-brown.com/ 鈥?FAXLine: (800) 548-6133
鈭單?/div>
CONVERTERS A LITTLE GAIN BOOST
WITH A FRONT END ANALOG GAIN STAGE
By Bonnie Baker
Sensing small signals is a challenge in process control
environments where the sensors used to capture tempera-
ture, pressure, and other environmental information produce
low level signals. These signals can easily be masked by the
ambient noise in the physical vicinity of the measurement,
placing tough requirements on the electronics in the remain-
der of the signal path. If a high level of accuracy is required
in these data acquisition systems, the effective signal full-
scale range must be decreased making the signal-to-noise
ratio (SNR) higher. Several techniques, in the digital domain
as well as analog domain, can be used to improve the signal-
to-noise ratio of the sensing system. An example of a circuit
that can easily implement digital and analog techniques to
improve the signal to noise ratio is shown in Figure 1. In this
circuit an instrumentation amplifier is used at the front end
of the analog signal path. This analog gain stage directly
changes the effective full scale range of the overall system.
Following the instrumentation amplifier, a delta-sigma con-
verter is utilized because of its internal analog gain and
digital filtering capability. When the delta-sigma converter
is properly configure, analog gain and digital filtering are
optimized to further improve the SNR. The finishing touches
of the signal conditioning is performed by the
碌Controller,
where software gain or bit shifting is used.
R
1
5.36k鈩?/div>
R
2
698鈩?/div>
R
3
357鈩?/div>
1
This application bulletin discusses the design trade-offs in
the implementation of an analog gain stage prior to the delta-
sigma converter. For this example an instrumentation ampli-
fier, such as the INA128 is used. The INA128 is chosen
because the inputs to the device are differential providing
good common-mode rejection over frequency and it can be
used in a single supply environment. The value of additional
gain prior to the A/D converter will be shown in a tempera-
ture sensing application circuit. For more information con-
cerning software gain, refer to Application Bulletin AB-106.
For a detailed discussion concerning the issues of imple-
menting the programmable gain amplifier and digital filter
of the delta sigma converter, refer to Application Bulletin
AB-108.
Sensing devices, such as thermocouples, RTDs and strain
gages respond to physical or mechanical occurrences that
are measured and manipulated in the process control envi-
ronment. The sensitivity of these devices vary and are
dependent on their construction. For example, the thermo-
couple (Figure 2a) is used to sense large temperature changes
in the ranges of 0擄C to 1700擄C. The thermocouple is
constructed with two dissimilar metals that are connected
together. A voltage is created as a result of the temperature
3
4
5
6
2.37k鈩?/div>
+5V
1N4148
Chromel
K-Type
Thermo-Couple
R
G
1.27k鈩?/div>
+In
4
Alumel
Copper
5
6
7
6pF
6pF
8
9
鈥揑n
V
REF
1
PGA
INA128
2
3
Voltage
Attenuator
碌Controller
Instruction Register
Command Register
Data Output Register
Offset Register
Full-Scale Register
Clock
鈭單?/div>
ADC
18
Voltage
REF
17
16
15
14
13
Serial
Interface
12
11
ADS1212
10
DVDD
+5V
FIGURE 1. This Circuit Implements a High Accuracy Thermocouple Interface with Cold Junction Compensation. The
鈭單?/div>
A/D
converter, ADS1212, is used to achieve high accuracy for this circuit in the 19- to 20-bit range. The INA128 is used
to match the output range of the thermocouple to the input full-scale range of the data conversion system.
漏
1996 Burr-Brown Corporation
AB-107
Printed in U.S.A. January, 1997
AB-107相關(guān)型號PDF文件下載
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型號
版本
描述
廠商
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英文版
Application Bulletin AB-12
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Application Bulletin AB-12
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AB-103 - NOISE ANALYSIS FOR HIGH SPEED OP AMPS
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AB-105 - TUNING IN AMPLIFIERS
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AB-109 - ADS7809 TAG FEATURE
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AB-110 - VOLTAGE REFERENCE SCALING TECHNIQUES Increase the A...
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AB-111 - DEM-ADS1210
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AB-113 - ACCESSING THE ADS1210 DEMO BOARD WITH YOUR PC
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AB-115 - OVERDRIVING THE INPUTS TO THE ADS1210. ADS1211. ADS...
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AB-118 - EXTRACT AND DIGITIZE AC SIGNALS WITH A SINGLE A
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＂AB-119 - A ＂＂GETTING STARTED＂＂ GUIDE FOR THE CONVERTER...
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