how gain impacts adc fsr, noise, and dynamic range
TRANSCRIPT
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How gain impacts ADC FSR, noise, and dynamic range TIPL 4256 TI Precision Labs โ ADCs
Created by Chris Hall & Bryan Lizon
Presented by Alex Smith
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ADC full-scale range (FSR)
ADS124S08 FSR (24-bit delta-sigma ADC)
ADS8691 FSR (18-bit SAR ADC)
ADS8900B FSR (20-bit SAR ADC)
ADC w/ no integrated gain ADCs w/ integrated gain
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ADS124S08 PGA input and output range
= ยฑ0.75 V
= ยฑ1.25 V
VREF
VSIG_OUT
FSROUT
= 2.5 V
= ยฑ1.5 V
= ยฑ2.5 V Gain = 2 V/V
๐น๐๐ ๐ด๐ท๐124๐08 = ยฑ๐๐ ๐ธ๐น
๐บ๐๐๐
60%
PGA
INPUT
PGA
OUTPUT
Refer both signal and
noise to the input
GAMP
VAINP
VAINN VOUTN
VOUTP
To access this calculator, navigate to the ADS124S08โs product folder on TI.com
VSIG_IN
FSRIN
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Output- versus input-referred noise
VN,RTI is the systemโs input resolution:
โข If VIN < VN,RTI, the signal is below
the noise floor
โข Else if VIN > VN,RTI, the signal can be
observed
For ADC w/ no gain, VN,RTO = VN,RTI = VN,ADC
Ideal ADC +
Equivalent ADC Noise Model:
VN,ADC
VN,RTO
VIN
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Input-referred noise for amp + ADC
For ADC w/ gain, VN,RTO โ VN,RTI
ADC +
Equivalent Amp + ADC Noise Model:
VN,RTI
VN,RTO
VIN GAMP
Ideal ADC Ideal
Amplifier
๐๐,๐ ๐๐ผ = (๐๐,๐ด๐๐(๐ ๐๐ผ))2 + ๐๐,๐ด๐ท๐ถ/๐บ๐ด๐๐2
๐
GAMP*VN,AMP(RTI) >>VN,ADC
๐๐,๐ ๐๐ = (๐๐,๐ด๐๐(๐ ๐๐ผ)โ ๐บ๐ด๐๐)2 + ๐๐,๐ด๐ท๐ถ2
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Input-referred noise for 2x amps + ADC
๐๐,๐ ๐๐ผ = ๐๐,๐ด๐๐1(๐ ๐๐ผ)2 +
๐๐,๐ด๐๐2(๐ ๐๐ผ)
๐บ๐ด๐๐1
2
+๐๐,๐ด๐ท๐ถ
๐บ๐ด๐๐1 โ. ๐บ๐ด๐๐2
2
๐ ๐
If GAMP1*GAMP2*VN,AMP1 (RTI) >> (GAMP2*VN,AMP2 (RTI))+VN,ADC, then VN,RTI = VN,AMP1(RTI)
Equivalent 2x Amplifier + ADC Noise Model:
+
VN,RTI
VIN VN,RTO
GAMP1
Ideal ADC Ideal
Amp 1
GAMP2
Ideal
Amp 2
ADC
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Lower vs higher-resolution ADC total noise
๐ฝ๐ต,๐ฎ=๐๐
๐ฝ๐ต,๐ฎ=๐๐๐=
๐. ๐ ยต๐ฝ๐น๐ด๐บ
๐. ๐ ยต๐ฝ๐น๐ด๐บ= ๐
๐ฝ๐ต,๐ฎ=๐๐
๐ฝ๐ต,๐ฎ=๐๐๐=
๐. ๐ ยต๐ฝ๐น๐ด๐บ
๐. ๐๐ ยต๐ฝ๐น๐ด๐บ= ๐. ๐
Parameters
(Sinc 3, 60 SPS)
Gain Units
1 2 4 8 16 32 64 128
Noise, RTI 1.4 0.7 0.37 0.21 0.12 0.11 0.1 0.089 ยตVRMS
Parameters
(Sinc 3, 60 SPS)
Gain Units
1 2 4 8 16 32 64 128
Noise, RTI 76.3 38.1 19.1 9.5 4.8 2.4 1.2 0.6 ยตVRMS
16-bit ADS114S08
24-bit ADS124S08
๐ฝ๐ต,๐ฎ=๐
๐ฝ๐ต,๐ฎ=๐=
๐. ๐ ยต๐ฝ๐น๐ด๐บ
๐. ๐ ยต๐ฝ๐น๐ด๐บ= ๐
๐ฝ๐ต,๐ฎ=๐
๐ฝ๐ต,๐ฎ=๐=
๐๐. ๐ ยต๐ฝ๐น๐ด๐บ
๐๐. ๐ ยต๐ฝ๐น๐ด๐บ= ๐
ADS114S08
ADS124S08
ADS1x4S08 block diagram
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Applying the input-referred noise equation
๐๐,๐ ๐๐ผ = (๐๐,๐ด๐๐(๐ ๐๐ผ))2 + ๐๐,๐ด๐ท๐ถ/๐บ๐ด๐๐2
(๐๐๐ ๐๐)
GAMP*VN,AMP(RTI) < VN,ADC
Lower-resolution ADCs โ
quantization noise dominates
GAMP*VN,AMP(RTI) >> VN,ADC
Higher-resolution ADCs โ
thermal noise dominates
โข Use a higher-noise (lower $) amp
โข Larger gain if system allows
โข Higher gain does not reduce noise
โข Use a very low noise amp
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How gain affects dynamic range (effective resolution)
Parameters
(Sinc 3, 60 SPS)
Gain
1 2 4 8 16 32 64 128
Full-scale range
(VREF = 2.5 V) ยฑ2.5 ยฑ1.25 ยฑ0.625 ยฑ0.313 ยฑ0.156 ยฑ0.078 ยฑ0.039 ยฑ0.019
Noise, RTI
(ยตVRMS) 1.4 0.7 0.37 0.21 0.12 0.11 0.1 0.089
Effective
resolution (bits) 21.8 21.8 21.7 21.5 21.3 20.4 19.5 18.7
Parameters
(Sinc 3, 60 SPS)
Gain
1 2 4 8 16 32 64 128
Full-scale range
(VREF = 2.5 V) ยฑ2.5 ยฑ1.25 ยฑ0.625 ยฑ0.313 ยฑ0.156 ยฑ0.078 ยฑ0.039 ยฑ0.019
Noise, RTI
(ยตVRMS) 76.3 38.1 19.1 9.5 4.8 2.4 1.2 0.6
Effective
resolution (bits) 16 16 16 16 16 16 16 16
16-bit ADS114S08 24-bit ADS124S08
Increasing gain
Constant effective resolution
Increasing gain
Decreasing effective resolution
๐ท๐ฆ๐๐๐๐๐ ๐๐๐๐๐ (๐๐๐๐๐๐ก๐๐ฃ๐ ๐๐๐ ๐๐๐ข๐ก๐๐๐) = ๐๐๐2๐น๐๐ ๐ ๐๐
๐๐,๐ ๐๐ (bits)
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Dynamic range: max vs system (low resolution ADC)
7.5
8.5
9.5
10.5
11.5
12.5
13.5
14.5
15.5
16.5
1 2 4 8 16 32 64 128
Useful gain limit =
limited by FSR
Dyn
am
ic r
an
ge (
bit
s)
ADS114S08 (16-bit) dynamic range vs gain (VREF = 2.5V)
Gain (V/V)
๐๐ผ๐ = ๐น๐๐ = ยฑ๐๐ ๐ธ๐น
๐บ๐๐๐
๐๐ผ๐ = 10 ๐๐
๐๐ฆ๐ ๐ก๐๐ ๐ท๐
= ๐๐๐2๐๐ผ๐,๐ ๐๐ (๐ ๐๐ผ)
๐๐,๐ ๐๐ (bits)
๐๐๐ฅ๐๐๐ข๐ ๐ท๐
= ๐๐๐2๐น๐๐ ๐ ๐๐
๐๐,๐ ๐๐ (bits)
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Dynamic range: max vs system (high resolution ADC)
13.5
14.5
15.5
16.5
17.5
18.5
19.5
20.5
21.5
22.5
1 2 4 8 16 32 64 128
Gain (V/V)
Dyn
am
ic r
an
ge (
bit
s)
Useful gain limit =
limited by amp noise
๐๐ผ๐ = ๐น๐๐ = ยฑ๐๐ ๐ธ๐น
๐บ๐๐๐
๐๐ผ๐ = 10 ๐๐
๐๐ฆ๐ ๐ก๐๐ ๐ท๐
= ๐๐๐2๐๐ผ๐,๐ ๐๐ (๐ ๐๐ผ)
๐๐,๐ ๐๐ (bits)
๐๐๐ฅ๐๐๐ข๐ ๐ท๐
= ๐๐๐2๐น๐๐ ๐ ๐๐
๐๐,๐ ๐๐ (bits)
ADS124S08 (24-bit) dynamic range vs gain (VREF = 2.5V)
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How gain affects dynamic range (SNR)
ADS86x1 datasheet SNR values (dB)
FSR ADS8661
(12-bit)
ADS8671
(14-bit)
ADS8681
(16-bit)
ADS8691
(18-bit)
ยฑ3 * VREF 73.5 84.5 92 92.5
ยฑ2.5 * VREF 73.5 84.5 92 92.5
ยฑ1.5 * VREF 73.5 84.25 91.5 91.5
ยฑ1.25 * VREF 73.5 84.25 91.5 91.5
ยฑ0.625 * VREF 73.5 84 90 90
Increasing
gain
Constant
SNR
SNR
decreases
by 2 dB
SNR
decreases
by 2.5 dB
SNR
decreases
by 0.5 dB
ADS86x1 block diagram
Increasing resolution
๐ท๐ฆ๐๐๐๐๐ ๐๐๐๐๐ ๐๐๐ = 20 โ ๐๐๐10๐น๐๐ ๐ ๐๐
๐๐,๐ ๐๐ (dB)
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Thanks for your time! Please try the quiz.
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1. When is an external amplifier most effective at improving the system noise
performance?
a) For lower resolution devices
b) For higher resolution devices
c) Using an amplifier cannot improve noise performance.
Quiz: How gain impacts ADC FSR, noise & DR
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Quiz: How gain impacts ADC FSR, noise & DR
1. When is an external amplifier most effective at improving the system noise
performance?
a) For lower resolution devices
b) For higher resolution devices
c) Using an amplifier cannot improve noise performance.
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2. When will increasing the gain of an amplifier driving an ADC cause system
noise RTI to decrease?
a) When amplifier noise is the dominant noise source.
b) When ADC noise is the dominant noise source.
c) Increasing gain will always decrease system noise RTI
d) Increasing gain will never decrease system noise RTI
Quiz: How gain impacts ADC FSR, noise & DR
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Quiz: How gain impacts ADC FSR, noise & DR
2. When will increasing the gain of an amplifier driving an ADC cause system
noise RTI to decrease?
a) When amplifier noise is the dominant noise source.
b) When ADC noise is the dominant noise source.
c) Increasing gain will always decrease system noise RTI
d) Increasing gain will never decrease system noise RTI
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3. In the table below, the effective resolution is approximately the same for gains
of 1V/V to 16V/V. For gains of 32V/V and higher, the effective resolution
drops off quickly. Which of the following statements is not true.
a) For the low gain ranges the ADC noise is dominant, so the ratio of FSR and noise
remain the same.
b) For higher gain ranges the amplifier noise is dominant, so FSR decreases but noise
stays constant.
c) For higher gain ranges the ADC noise is dominant causing the effective resolution to
decrease.
Quiz: How gain impacts ADC FSR, noise & DR
Parameters
(Sinc 3, 60 SPS)
Gain
1 2 4 8 16 32 64 128
Full-scale range
(VREF = 2.5 V) ยฑ2.5 ยฑ1.25 ยฑ0.625 ยฑ0.313 ยฑ0.156 ยฑ0.078 ยฑ0.039 ยฑ0.019
Noise, RTI
(ยตVRMS) 1.4 0.7 0.37 0.21 0.12 0.11 0.1 0.089
Effective
resolution (bits) 21.8 21.8 21.7 21.5 21.3 20.4 19.5 18.7
๐ธ๐๐๐๐๐ก๐๐ฃ๐ ๐๐๐ ๐๐๐ข๐ก๐๐๐ = ๐๐๐2๐น๐๐ ๐ ๐๐
๐๐,๐ ๐๐ (bits)
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3. In the table below, the effective resolution is approximately the same for gains
of 1V/V to 16V/V. For gains of 32V/V and higher, the effective resolution
drops off quickly. Which of the following statements is not true.
a) For the low gain ranges the ADC noise is dominant, so the ratio of FSR and noise
remain the same.
b) For higher gain ranges the amplifier noise is dominant, so FSR decreases but noise
stays constant.
c) For higher gain ranges the ADC noise is dominant causing the effective resolution to
decrease.
Quiz: How gain impacts ADC FSR, noise & DR
Parameters
(Sinc 3, 60 SPS)
Gain
1 2 4 8 16 32 64 128
Full-scale range
(VREF = 2.5 V) ยฑ2.5 ยฑ1.25 ยฑ0.625 ยฑ0.313 ยฑ0.156 ยฑ0.078 ยฑ0.039 ยฑ0.019
Noise, RTI
(ยตVRMS) 1.4 0.7 0.37 0.21 0.12 0.11 0.1 0.089
Effective
resolution (bits) 21.8 21.8 21.7 21.5 21.3 20.4 19.5 18.7
๐ธ๐๐๐๐๐ก๐๐ฃ๐ ๐๐๐ ๐๐๐ข๐ก๐๐๐ = ๐๐๐2๐น๐๐ ๐ ๐๐
๐๐,๐ ๐๐ (bits)