Datasheet ADS1015

33
ADS1013 , ADS1014 , ADS1015
www.ti.com SBAS473E –MAY 2009–REVISED JANUARY 2018
Product Folder Links: ADS1013 ADS1014 ADS1015
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9.2.2.1 Shunt Resistor Considerations
A shunt resistor (R
SHUNT
) is an accurate resistance inserted in series with the load as shown in Figure 29. If the
absolute voltage drop across the shunt, |V
SHUNT
|, is a larger percentage of the bus voltage, the voltage drop may
reduce the overall efficiency and system performance. If |V
SHUNT
| is too low, measuring the small voltage drop
requires careful design attention and proper selection of the ADC, operation amplifier, and precision resistors.
Make sure that the absolute voltage at the shunt terminals does not result in violation of the input common-mode
voltage range requirements of the operational amplifier. The power dissipation on the shunt resistor increases
the temperature because of the current flowing through it. To minimize the measurement errors due to variation
in temperature, select a low-drift shunt resistor. To minimize the measurement gain error, select a shunt resistor
with low tolerance value. To remove the errors due to stray ground resistance, use a four-wire Kelvin-connected
shunt resistor, as shown in Figure 29.
9.2.2.2 Operational Amplifier Considerations
The operational amplifier used for this design example requires the following features:
• Unipolar supply operation (5 V)
• Low input offset voltage (< 10 µV) and input offset voltage drift (< 0.5 µV/°C)
• Rail-to-rail input and output capability
• Low thermal and flicker noise
• High common-mode rejection (> 100 dB)
OPA333 offers all these benefits and is selected for this application.
9.2.2.3 ADC Input Common-Mode Considerations
V
CM
sets the V
OUT
common-mode voltage by appropriate selection of precision resistors R
1
, R
2
, R
3
, and R
4
.
If R
1
= R
3
, R
2
= R
4
, and V
SHUNT
= 0 V, V
OUT
is given by Equation 8.
V
OUT
= V
CM
(8)
If V
OUT
is connected to the ADC positive input (AINP) and V
CM
is connected to the ADC negative input (AINN),
V
CM
appears as a common-mode voltage to the ADC. This configuration allows pseudo-differential
measurements and uses the maximum dynamic range of the ADC if V
CM
is set at midsupply (VDD / 2). A resistor
divider from VDD to GND followed by a buffer amplifier can be used to generate V
CM
.
9.2.2.4 Resistor (R
1
, R
2
, R
3
, R
4
) Considerations
Proper selection of resistors R
1
, R
2
, R
3
and R
4
is critical for meeting the overall accuracy requirements.
Using Equation 6, the offset term, V
OUT-OS
, and the gain term, A
OUT
, of the differential ADC input are represented
by Equation 9 and Equation 10 respectively. The error contributions from the first-order RC filters are ignored.
V
OUT-OS
= V
CM
· (R
2
/ R
1
- R
3
/ R
4
) / (1 + R
3
/ R
4
) (9)
A
OUT
= (1 + R
2
/ R
1
) / (1 + R
4
/ R
3
) (10)
The tolerance, drift and linearity performance of these resistors is critical to meeting the overall accuracy
requirements. In Equation 9, if R
1
= R
3
and R
2
= R
4
, V
OUT-OS
= 0 V and therefore, the common-mode voltage,
V
CM
, only contributes to level-shift V
SHUNT
and does not introduce any error at the differential ADC inputs. High-
precision resistors provide better common-mode rejection from V
CM
.
9.2.2.5 Noise and Input Impedance Considerations
If v
n_res
represents the input-referred rms noise from all the resistors, v
n_op
represents the input-referred rms
noise of OPA333, and v
n_ADC
represents the input-referred rms noise of ADS1015, the total input-referred noise
of the entire system, v
N
, can be approximated by Equation 11.
v
N
2
= v
n_res
2
+ v
n_op
2
+ v
n_ADC
/ (1 + R
2
/ R
1
)
2
(11)
It is important to note that the ADC noise contribution, v
n_ADC
, is attenuated by the non-inverting gain stage.