Download Quiz: Linear Range ADC + FDA

Quiz: Driving a SAR ADC
with a Fully Differential Amplifier
TIPL 4103
TI Precision Labs – ADCs
Created by Art Kay
1
Quiz: Linear Range ADC + FDA
1. Design a FDA circuit topology to translate a single ended 0V to 1V, to 0V to
4V. Use THS4551, OPA320, and ADS9110 with a 4.096V reference. Will the
output be linear over the full 0V to 1V signal range?
2. Design a FDA circuit topology to translate a single ended -0.1V to +0.1V, to 0V
to 2.5V. Use THS4551, OPA320, and ADS9110 with a 2.5V reference. Will
the output be linear over the full 0V to 1V signal range?
3. (T/F) Assume the input signal applied to an FDA has both even and odd order
harmonics. The the FDA will cancel the even order harmonics.
4. (T/F) The Vocm pin can be directly connected to a voltage divider to set it’s
output common mode.
2
Solutions
3
Quiz: Linear Range ADC + FDA
1. Design a FDA circuit topology to translate a single ended 0V to 1V, to 0V to
4V. Use THS4551, OPA320, and ADS9110 with a 4.096V reference. Will the
output be linear over the full 0V to 1V signal range?
2.048V
Buffer
U3
High BW
10k
4.096V
1.8V
1.8V
Vref
AVDD
DVDD
10k
RF1 2k
RG1 1k
2.048V
AIN_P
5V
Connect to
Vref/2 for
Unipolar
RG3 6.04k
2.048V
RF3 18.7k
5V
+
VOCM
+
PD
+
-
U1
THS4551
+
-+
Vin
0V to 1V
+
Vout_dif = ±4.096V
Vcm = 2.048V
ADS9110
AIN_M
AGND
RG2 1k
U2
OPA320
RF2 2k
0V to 4.096V
DGND
Analog engineer’s
calculator used to select U2
Feedback resistors
4
Quiz: Linear Range ADC + FDA
Problem 1: continued.
PARAMETER OPA320
INPUT VOLTAGE
Common-mode voltage range
OUTPUT
Voltage swing from both rails
TEST CONDITION
MIN
Vcm
VO
TYP
MAX
(V+)+0.1
(V-) - 0.1
RL = 10kΩ
10
20
RL = 2kΩ
25
35
UNIT
V
mV
OPEN-LOOP GAIN
Amplifier input range
Amplifier output range
AOL
-0.1V < Vcm < 5.1V
0.02 < VO < 4.98V
Amplifier Linear Range 0.1 < VO < 4.9V
Worst Case Range
0.1 < VO < 4.9V
114
108
RG3 6.04k
132
123
dB
RF3 18.7k
5V
-+
+
Open-loop gain
0.1 < Vo < (V+)-0.1V, RL = 10kΩ
0.2 < Vo < (V+)-0.2V, RL = 2kΩ
Vin
0V to 1V
+
U2
OPA320
Target:0V to 4.096V
Linear range: 0.1V to 4.096V
5
Quiz: Linear Range ADC + FDA
Problem 1: continued.
PARAMETER THS4551
MIN
Output voltage low
(V+) – 0.23
Output voltage high
(V+) – 1.2
V
(V-) – 0.1
2.048V
VOCM
+
PD
output range
0.23 < VO < 4.77V
Fails output range.
Must be > 0.23V
Maximum output swing
RF1 2k
RF1 2k
RG1 1k
2.048V
0V
+
-
2.048V
Vout_dif = -3.996V
Vcm = 2.048V
3.996V
RF2 2k
4.096V - 0.1V = 3.996V
VOCM
+
U1
THS4551
+
PD
5V
+
-
-
Vout_dif = -4.096V
Vcm = 2.048V
2.048V
4.096V
RF2 2k
Vcm_in = 1.365V
Passes input range
PD
+
-
Vout_dif = +4.096V
Vcm = 2.048V
U1
THS4551
4.096V
0V
RG2 1k
Passes output
range
+
VOCM
+
U1
THS4551
0V
RG2 1k
4.096V
2.048V
5V
0.1V
RG2 1k
-0.1V < Vcm < 3.8V
V
5V
+
input range
V
RG1 1k
0.1V
-
V
Ideal minimum output swing
RF1 2k
2.048V
UNIT
(V+) – 1.1
Minimum output swing from OpAmp
RG1 1k
MAX
(V-) + 0.23
(V+) – 0.2
(V-) + 0.2
Common mode input voltage low
Common mode input voltage high
TYP
(V-) + 0.2
RF2 2k
Vcm_in = 2.731V
Passes input range
Fails output range.
Must be > 0.23V
Note: The output swing low on the THS limits the range.
6
Quiz: Linear Range ADC + FDA
Problem 1: continued.
2.048V
Buffer
U3
High BW
10k
4.096V
1.8V
1.8V
Vref
AVDD
DVDD
10k
RF1 2k
RG1 1k
2.048V
AIN_P
5V
Connect to
Vref/2 for
Unipolar
RG3 6.04k
2.048V
RF3 18.7k
5V
+
VOCM
+
PD
+
-
U1
THS4551
-0.3V
+
-+
Vin
0V to 1V
+
RG2 1k
U2
OPA320
-0.3V
Vout_dif = ±4.096V
Vcm = 2.048V
ADS9110
AIN_M
AGND
DGND
RF2 2k
0V to 4.096V
Note: One approach to avoiding the output swing limitation is a -0.3V negative supply.
7
Quiz: Linear Range ADC + FDA
2. Design a FDA circuit topology to translate a single ended -0.1V to +0.1V, to 0V to
2.5V. Use THS4551, OPA320, and ADS9110 with a 2.5V reference. Will the
output be linear over the full 0V to 1V signal range?
1.25V
U3
Buffer
10k
2.5V
1.8V
1.8V
Vref
AVDD
DVDD
10k
RF1 2k
RG1 1k
AIN_P
5V
1.25V
RG3 1k
RF3 11.5k
+2.5V
+
VOCM
+
PD
+
-
U1
THS4551
+
-+
Vin
-0.1V to 0.1V
+
Vout_dif = ±2.5V
Vcm = 1.25V
RG2 1k
RF2 2k
U2
OPA320
-2.5V
-1.25V to 1.25V
ADS9110
AIN_M
AGND
DGND
Analog engineer’s
calculator used to select
U2 Feedback resistors
8
Quiz: Linear Range ADC + FDA
Problem 2: continued.
PARAMETER OPA320
INPUT VOLTAGE
Common-mode voltage range
OUTPUT
Voltage swing from both rails
TEST CONDITION
Vcm
VO
MIN
TYP
MAX
(V+)+0.1
(V-) - 0.1
RL = 10kΩ
10
20
RL = 2kΩ
25
35
UNIT
V
mV
OPEN-LOOP GAIN
Open-loop gain
Amplifier input range
Amplifier output range
AOL
0.1 < Vo < (V+)-0.1V, RL = 10kΩ
0.2 < Vo < (V+)-0.2V, RL = 2kΩ
-2.6V < Vcm < 2.6V
-2.48 < VO < 2.48V
114
108
RG3 1k
132
123
dB
RF3 11.5k
+2.5V
Amplifier Linear Range -2.4 < VO < 2.4V
Worst Case Range
-2.4 < VO < 2.4V
+
-+
Vin
-0.1V to 0.1V
+
U2
OPA320
-2.5V
Target:-1.25V to +1.25V
Worst Case range: -2.4V to +2.4V
Not limited
9
Quiz: Linear Range ADC + FDA
Problem 2: continued.
PARAMETER THS4551
MIN
TYP
(V-) + 0.2
Output voltage low
(V+) – 0.23
Output voltage high
Common mode input voltage high
(V+) – 0.2
(V-) + 0.2
Common mode input voltage low
(V+) – 1.2
Minimum output swing
MAX
(V-) + 0.23
UNIT
V
V
(V-) – 0.1
(V+) – 1.1
-0.1V < Vcm < 3.8V
output range
0.23 < VO < 4.77V
V
V
Fails output range.
Must be > 0.23V
Maximum output swing
RF1 2k
RG1 1k
input range
RF1 2k
RG1 1k
2.5V
0V
5V
1.25V
+
VOCM
+
PD
5V
+
-
Vout_dif = -2.5V
Vcm = 1.25V
1.25V
+
2.5V
RF2 2k
Vcm_in = 1.365V
Passes input range
PD
+
-
Vout_dif = +2.5V
Vcm = 1.25V
U1
THS4551
1.25V
0V
RG2 1k
Passes output
range
+
VOCM
U1
THS4551
-1.25V
RG2 1k
-
RF2 2k
Vcm_in = 2.731V
Passes input range
Fails output range.
Must be > 0.23V
10
Quiz: Linear Range ADC + FDA
Problem 2: continued.
1.25V
U3
Buffer
10k
2.5V
1.8V
1.8V
Vref
AVDD
DVDD
10k
RF1 2k
RG1 1k
AIN_P
5V
1.25V
RG3 1k
RF3 11.5k
+2.5V
+
VOCM
+
PD
+
-
U1
THS4551
-0.3V
+
-+
Vin
-0.1V to 0.1V
Vout_dif = ±2.5V
Vcm = 1.25V
ADS9110
AIN_M
AGND
DGND
+
RG2 1k
RF2 2k
U2
OPA320
-2.5V
-1.25V to 1.25V
Note: One approach to avoiding the output swing limitation is a -0.3V negative supply.
11
Quiz: Linear Range ADC + FDA
3. (False) Assume the input signal applied to an FDA has both even and odd
order harmonics. The the FDA will cancel the even order harmonics.
4. (False) The Vocm pin can be directly connected to a voltage divider to set it’s
output common mode.
12