SAR ADC Input Types
TIPL 4003
TI Precision Labs – ADCs
Created by Art Kay
Presented by Peggy Liska
1
Single Ended
Pseudo-differential
3V
2V
3V
2V
AVDD
Vref
DVDD
AVDD
Vref
DVDD
AIN_P
Vin
-0.1V to 3.1V FSR
0V to 3.0V
AIN_P
Vin
-0.1V to 3.1V
ADS7042
FSR
±1.5V
ADS7043
AIN_M
AIN_M
Vin
-0.1V to +0.1V
AGND
DGND
Vin dif = AIN_P – AIN_M
Vin dif = 0.0V – 0.0V = 0.0V
3.0
AIN_P
2.25
0.0
Vin dif = AIN_P – AIN_M
Vin dif = 0V – 1.5V = -1.5V
AIN_P
2.25
1.5
1.5
0.75
DGND
Vin dif = AIN_P – AIN_M
Vin dif = 3.0V – 1.5V = +1.5V
Vin dif = AIN_P – AIN_M
Vin dif = 3.0V – 0.0V = +3.0V
3.0
AGND
Vin = ½ FSR ±0.1V
1.5V
AIN_M ≈ 0V
Time (sec)
0.75
0.0
AIN_M = ½ FSR = 1.5V
Time (sec)
2
ADC Input Swing and Common Mode
Input to ADC
Vdif = AIN_P - AIN_M
5.0
AIN_M
3.75
2.5
Vout_dif = 4.9V – 0.1V = 4.8V
1.25
Vout_dif = 0V – 5V = -5V
AIN_P
•
•
•
•
0.0
Differential of ±5V (10Vpp)
Single ended equivalent 0V to 5V (5Vpp)
Double the range of single ended
Negative differential output can occur when
absolute output voltage of each output is
positive (unipolar single supply)
Input to ADC
5.0
AIN_M
Vcm =
3.75
2.5
1.25
AIN_P + AIN_M
2
3.75V + 1.25V
= 2.5V
Vcm =
2
•
In this example common mode is always 2.5V
AIN_P
0.0
3
Fully Differential Input
PARAMETER 9110
MIN
TYP
MAX
UNIT
ADC Input (Fully Differential)
ANALOG INPUT
-Vref
Absolute Input voltage range
AIN_P to GND -0.1
Vref + 0.1
AIN_M to GND -0.1
Vref + 0.1
(Vref/2)-0.1
Common-mode voltage range
(AIN_P + AIN_M)/2
Vref/2
V
(Vref/2)+0.1
5V
1.8V
ADC Input Voltage
Full-scale input voltage span
5
Vref
AIN_M
4
3
2
1
AIN_P
0
1.8V
0
0.5
1
1.5
2
2.5
3
Time
Vcm must be
constant at Vref/2
Vref
AVDD
DVDD
ADC Input Viltage
Vcm for Fully Diff Example
Vin
-0.1V to 5.1V
FSR
±5V
ADS9110
AIN_M
Vin
-0.1V to 5.1V
AGND
DGND
Common Mode Voltage (V)
AIN_P
5
4
Vcm =
3
AIN_P + AIN_M
2
2
1
0
0
0.5
1
1.5
2
2.5
3
Time
4
True Differential Input
PARAMETER ADS8881
MIN
TYP
MAX
UNIT
ADC Input Voltages (True Differential)
ANALOG INPUT
-Vref
Absolute Input voltage range
AIN_P to GND -0.1
Vref + 0.1
AIN_M to GND -0.1
Vref + 0.1
0.0
Common-mode voltage range
(AIN_P + AIN_M)/2
Vref/2
V
Vref
ADC Input Voltage
Full-scale input voltage span
5
Vref
AIN_M
4
3
2
1
AIN_P
0
Vcm has a wide
voltage range.
5V
3V
3V
Vref
AVDD
DVDD
0
AGND
DGND
Common Mode Voltage (V)
5
AIN_M
Vin
-0.1V to 5.1V
1.5
2
2.5
3
2.5
3
Vcm for True Diff Example
ADS8881
FSR
±5V
1
Time
AIN_P
Vin
-0.1V to 5.1V
0.5
Vcm =
4
AIN_P + AIN_M
2
3
2
1
0
0
0.5
1
1.5
2
Time
5
Summary of ADC input types
Input Type
Vref
Ain_P
Ain_M
Vcm
FSR
Example 16 Bit Output
Single Ended
5V
0V to 5V
GND (+/- 100mV)
n/a
0V to 5V
Pseudo
Differential
Fully
Differential
True
Differential
5V
0V to 5V
Set ½ * Range (2.5V)
n/a
-2.5V to +2.5V
5V
0V to 5V
0V to 5V
-5V to +5V
5V
0V to 5V
0V to 5V
Set ½ * Range
(2.5V)
Can vary from
–FS to +FS
0000 = 0V
FFFF = FSR = 5V
8000 = -2.5V
7FFF = +2.5V
8000 = -5V
7FFF = +5V
8000 = -5V
7FFF = +5V
-5V to +5V
6
Unipolar vs Bipolar
AIN_P
3V
2V
4.096V
5V
3V
AVDD
Vref
DVDD
Vref
AVDD
DVDD
AIN_P
AIN_P
Vin
0V to 3V
AIN_P
ADS7042
AIN_M
Vin
-10V to +10V
ADS8681
AIN_M
AGND
Unipolar Input
DGND
AGND
DGND
Bipolar Input
7
Unipolar with Differential Input
5V
3V
3V
Vref
AVDD
DVDD
3V
Vref
AVDD
DVDD
Transfer Function:
Unipolar with Differential Inputs
1FFFF
1FFFE
AIN_P
AIN_P
Vin
5V Vin_dif
+5V
ADS8881
AGND
DGND
Negative Full Scale Input
Vin_dif = AIN_P – AIN_M
Vin_dif = 0V - 5V = -5V
AGND
Vin
0V
DGND
1.0
Vin
5V
AIN_M
5.0
4.999961
4.999923
AIN_M
00002
00001
76.3E-6
38.1E-6
AIN_M
ADS8881
-4.999923
-4.999961
-5.0
AIN_M
3V
AIN_P
AIN_P
Vin
0V Vin_dif
-5V
5V
Positive Full Scale Input
20001
20000
Vin_dif = AIN_P – AIN_M
Vin_dif = 5V - 0V = +5V
8
Input Impedance: Resistive vs. Switched Capacitor
Switched Capacitor Input
Resistive, High Voltage, PGA input
5V
3V
AVDD
DVDD
5V
REF6050
5.0V
Reference
5V
S1
CSH
40pF
Vref AVDD DVDD
Input Range = 0 to Vref
Abs Max Input = Vref + 0.3V
Dynamic input impedance
Comp
+
OPA320
BW = 20MHz
N-Bit
Register
+15V
-
AGND
Input Range = ±12.288V
Abs Max Input = ±20V
1MΩ Resistive input impedance
Vref
1MΩ
OPA188
BW = 2MHz
PGA
1MΩ
ADC
Drive
•
•
•
RSH
50Ω
S1
CSH
40pF
Comp
+
N-Bit
Register
N-Bit
CDAC
Vbias
ADS8681
DGND
Wide bandwidth external amp required
Dynamic input impedance
Input voltage range set by reference
4.096V
Reference
+
-15V
N-Bit
CDAC
ADS8860
•
•
•
3V
RSH
50Ω
+
3V
AGND
DGND
External amplifier bandwidth not critical
Internal PGA, ADC driver, and reference
High voltage input (±12.288V) with 5V supply
9
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10