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Analog Dialogue 52-12, December 2018
1
Integrated PGIAs
ADI offers a number of integrated PGIAs in its portfolio. Integrated PGIAs
provide the benefit of shorter design time and a smaller footprint. The
digitally adjustable gain is achieved with internal precision resistor arrays.
On-chip trimming of these resistor arrays can be done to optimize gain,
CMRR, and offsets, resulting in good overall dc performance. Design tech-
niques can also be utilized for compact IC layout to minimize parasitics,
as well as provide excellent matching, resulting in good ac performance.
Because of these advantages, it is always recommended to choose an
integrated PGIA, if there is one that meets the design requirement. Table 1
lists available integrated PGIAs along with some key specifications.
The choice of PGIA is dependent on the application. The AD825x is very
useful in multiplexed systems due to its fast settling time and high slew
rate. The AD8231 and LTC6915, which have zero-drift architectures, are
useful for systems where precision performance is required over a wide
range of temperatures.
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Introduction
Data acquisition (DAQ) systems find their use in many industries, for a
wide range of applications, such as research, analysis, design verification,
manufacturing, and testing. By nature, these systems interface to various
sensors, which poses a challenge to the front end. Different sensor sensi-
tivities must be considered—for example, a system may need to interface
to a load sensor that has a maximum output of 10 mV and submicrovolt
sensitivity, while also interfacing to a sensor that is preconditioned for
a 10 V output. With a single gain, the system needs to have a very high
resolution to sense both inputs. Even then, signal-to-noise ratio (SNR) is
sacrificed at the lowest inputs.
In these applications, programmable gain instrumentation amplifiers
(PGIAs) are a good solution for the front end to accommodate the sensi-
tivities of the various sensor interfaces, while optimizing SNR. Integrated
PGIAs are available to achieve good dc and ac specifications. This article
discusses the various integrated PGIAs and the advantages in using them.
Limitations will also be discussed, along with guidelines for building a dis-
crete PGIA when trying to meet a specific requirement.
Programmable Gain Instrumentation
Ampliers: Finding One that Works for You
By Kristina Fortunado
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AD825x AD8231 LTC6915
Gain Settings
1, 2, 5, 10 (AD8250) 1, 2, 4, 8 (AD8251)
1, 10, 100, 1000 (AD8253)
1 to 128 in 6 dB steps 1 to 4096 in 6 dB steps
CMRR (G = 1) 80 dB 80 dB 125 dB
Gain Drift 10 ppm/°C 10 ppm/°C
Quiescent Current 4.5 mA 4 mA 2 mA
Bandwidth 10 MHz 2.7 MHz 200 kHz
Settling time 0.78 µs 4 µs
Offset Voltage (G=1) 1.05 mV 45 µV 10 µV>
Offset Voltage Drift 6.2 µV/°C 50 nV/°C 50 nV/°C
Input Bias Current 50 nA 500 pA 10 nA
Noise (G=1) 45 nV/√Hz 66 nV/√Hz 2.5 µV p-p (0.1 Hz to 10 Hz)
Gain Nonlinearity 6 ppm 3 ppm 15 ppm
Rail-to-Rail In No (–V
s
+1) to (+V
s
–1.5) Yes (0.2 V beyond the rails) Yes
Table 1. Programmable Gain Instrumentation Amplifiers Specifications
ADAS3022 ADAS3023 AD7124-8
Description 16-Bit, 1 MSPS, 8-channel DAQ system
16-Bit, 8-channel simultaneous sampling
DAQ system
8-Channel, low noise, low power, 24-bit
Σ-Δ ADC with PGA and reference
Gain Settings 0.16, 0.2, 0.4, 0.8, 1.6, 3.2, 6.4 0.2, 0.4, 0.8, 1.6 1 to 128 in 6 dB steps
CMRR (G=1) 90 dB 95 dB 85 dB
Gain Drift 0.1 ppm/°C 1 ppm/°C 2 ppm/°C
Power (Max Gain)
Conversion Rate (Max Channels)
12 mA
125 kSPS
10.5 mA
125 kSPS
1.2 mA
19.2 kSPS (full power)
Table 2. DAQ System Specifications
