1 1
How gain impacts ADC FSR, noise, and dynamic range TIPL 4256 TI Precision Labs – ADCs
Created by Chris Hall & Bryan Lizon
Presented by Alex Smith
2
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
3
ADS124S08 PGA input and output range
= ±0.75 V
= ±1.25 V
VREF
VSIG_OUT
FSROUT
= 2.5 V
= ±1.5 V
= ±2.5 V Gain = 2 V/V
𝐹𝑆𝑅𝐴𝐷𝑆124𝑆08 = ±𝑉𝑅𝐸𝐹
𝐺𝑎𝑖𝑛
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 ADS124S08’s product folder on TI.com
VSIG_IN
FSRIN
4
Output- versus input-referred noise
VN,RTI is the system’s input resolution:
• If VIN < VN,RTI, the signal is below
the noise floor
• 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
5
Input-referred noise for amp + ADC
For ADC w/ gain, VN,RTO ≠ VN,RTI
ADC +
Equivalent Amp + ADC Noise Model:
VN,RTI
VN,RTO
VIN GAMP
Ideal ADC Ideal
Amplifier
𝑉𝑁,𝑅𝑇𝐼 = (𝑉𝑁,𝐴𝑀𝑃(𝑅𝑇𝐼))2 + 𝑉𝑁,𝐴𝐷𝐶/𝐺𝐴𝑀𝑃2
𝟎
GAMP*VN,AMP(RTI) >>VN,ADC
𝑉𝑁,𝑅𝑇𝑂 = (𝑉𝑁,𝐴𝑀𝑃(𝑅𝑇𝐼)∗ 𝐺𝐴𝑀𝑃)2 + 𝑉𝑁,𝐴𝐷𝐶2
6
Input-referred noise for 2x amps + ADC
𝑉𝑁,𝑅𝑇𝐼 = 𝑉𝑁,𝐴𝑀𝑃1(𝑅𝑇𝐼)2 +
𝑉𝑁,𝐴𝑀𝑃2(𝑅𝑇𝐼)
𝐺𝐴𝑀𝑃1
2
+𝑉𝑁,𝐴𝐷𝐶
𝐺𝐴𝑀𝑃1 ∗. 𝐺𝐴𝑀𝑃2
2
𝟎 𝟎
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
7
Lower vs higher-resolution ADC total noise
𝑽𝑵,𝑮=𝟔𝟒
𝑽𝑵,𝑮=𝟏𝟐𝟖=
𝟏. 𝟐 µ𝑽𝑹𝑴𝑺
𝟎. 𝟔 µ𝑽𝑹𝑴𝑺= 𝟐
𝑽𝑵,𝑮=𝟔𝟒
𝑽𝑵,𝑮=𝟏𝟐𝟖=
𝟎. 𝟏 µ𝑽𝑹𝑴𝑺
𝟎. 𝟎𝟗 µ𝑽𝑹𝑴𝑺= 𝟏. 𝟏
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 µVRMS
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 µVRMS
16-bit ADS114S08
24-bit ADS124S08
𝑽𝑵,𝑮=𝟏
𝑽𝑵,𝑮=𝟐=
𝟏. 𝟒 µ𝑽𝑹𝑴𝑺
𝟎. 𝟕 µ𝑽𝑹𝑴𝑺= 𝟐
𝑽𝑵,𝑮=𝟏
𝑽𝑵,𝑮=𝟐=
𝟕𝟔. 𝟑 µ𝑽𝑹𝑴𝑺
𝟑𝟖. 𝟏 µ𝑽𝑹𝑴𝑺= 𝟐
ADS114S08
ADS124S08
ADS1x4S08 block diagram
8
Applying the input-referred noise equation
𝑉𝑁,𝑅𝑇𝐼 = (𝑉𝑁,𝐴𝑀𝑃(𝑅𝑇𝐼))2 + 𝑉𝑁,𝐴𝐷𝐶/𝐺𝐴𝑀𝑃2
(𝑛𝑉𝑅𝑀𝑆)
GAMP*VN,AMP(RTI) < VN,ADC
Lower-resolution ADCs –
quantization noise dominates
GAMP*VN,AMP(RTI) >> VN,ADC
Higher-resolution ADCs –
thermal noise dominates
• Use a higher-noise (lower $) amp
• Larger gain if system allows
• Higher gain does not reduce noise
• Use a very low noise amp
9
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) ±2.5 ±1.25 ±0.625 ±0.313 ±0.156 ±0.078 ±0.039 ±0.019
Noise, RTI
(µVRMS) 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) ±2.5 ±1.25 ±0.625 ±0.313 ±0.156 ±0.078 ±0.039 ±0.019
Noise, RTI
(µVRMS) 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
𝐷𝑦𝑛𝑎𝑚𝑖𝑐 𝑟𝑎𝑛𝑔𝑒 (𝑒𝑓𝑓𝑒𝑐𝑡𝑖𝑣𝑒 𝑟𝑒𝑠𝑜𝑙𝑢𝑡𝑖𝑜𝑛) = 𝑙𝑜𝑔2𝐹𝑆𝑅𝑅𝑀𝑆
𝑉𝑁,𝑅𝑀𝑆 (bits)
10
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)
𝑉𝐼𝑁 = 𝐹𝑆𝑅 = ±𝑉𝑅𝐸𝐹
𝐺𝑎𝑖𝑛
𝑉𝐼𝑁 = 10 𝑚𝑉
𝑆𝑦𝑠𝑡𝑒𝑚 𝐷𝑅
= 𝑙𝑜𝑔2𝑉𝐼𝑁,𝑅𝑀𝑆 (𝑅𝑇𝐼)
𝑉𝑁,𝑅𝑀𝑆 (bits)
𝑀𝑎𝑥𝑖𝑚𝑢𝑚 𝐷𝑅
= 𝑙𝑜𝑔2𝐹𝑆𝑅𝑅𝑀𝑆
𝑉𝑁,𝑅𝑀𝑆 (bits)
11
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
𝑉𝐼𝑁 = 𝐹𝑆𝑅 = ±𝑉𝑅𝐸𝐹
𝐺𝑎𝑖𝑛
𝑉𝐼𝑁 = 10 𝑚𝑉
𝑆𝑦𝑠𝑡𝑒𝑚 𝐷𝑅
= 𝑙𝑜𝑔2𝑉𝐼𝑁,𝑅𝑀𝑆 (𝑅𝑇𝐼)
𝑉𝑁,𝑅𝑀𝑆 (bits)
𝑀𝑎𝑥𝑖𝑚𝑢𝑚 𝐷𝑅
= 𝑙𝑜𝑔2𝐹𝑆𝑅𝑅𝑀𝑆
𝑉𝑁,𝑅𝑀𝑆 (bits)
ADS124S08 (24-bit) dynamic range vs gain (VREF = 2.5V)
12
How gain affects dynamic range (SNR)
ADS86x1 datasheet SNR values (dB)
FSR ADS8661
(12-bit)
ADS8671
(14-bit)
ADS8681
(16-bit)
ADS8691
(18-bit)
±3 * VREF 73.5 84.5 92 92.5
±2.5 * VREF 73.5 84.5 92 92.5
±1.5 * VREF 73.5 84.25 91.5 91.5
±1.25 * VREF 73.5 84.25 91.5 91.5
±0.625 * 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
𝐷𝑦𝑛𝑎𝑚𝑖𝑐 𝑟𝑎𝑛𝑔𝑒 𝑆𝑁𝑅 = 20 ∗ 𝑙𝑜𝑔10𝐹𝑆𝑅𝑅𝑀𝑆
𝑉𝑁,𝑅𝑀𝑆 (dB)
13
Thanks for your time! Please try the quiz.
14
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
15
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.
16
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
17
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
18
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) ±2.5 ±1.25 ±0.625 ±0.313 ±0.156 ±0.078 ±0.039 ±0.019
Noise, RTI
(µVRMS) 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
𝐸𝑓𝑓𝑒𝑐𝑡𝑖𝑣𝑒 𝑟𝑒𝑠𝑜𝑙𝑢𝑡𝑖𝑜𝑛 = 𝑙𝑜𝑔2𝐹𝑆𝑅𝑅𝑀𝑆
𝑉𝑁,𝑅𝑀𝑆 (bits)
19
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) ±2.5 ±1.25 ±0.625 ±0.313 ±0.156 ±0.078 ±0.039 ±0.019
Noise, RTI
(µVRMS) 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
𝐸𝑓𝑓𝑒𝑐𝑡𝑖𝑣𝑒 𝑟𝑒𝑠𝑜𝑙𝑢𝑡𝑖𝑜𝑛 = 𝑙𝑜𝑔2𝐹𝑆𝑅𝑅𝑀𝑆
𝑉𝑁,𝑅𝑀𝑆 (bits)