Datasheet ADS1015

34
ADS1013 , ADS1014 , ADS1015
SBAS473E –MAY 2009–REVISED JANUARY 2018 www.ti.com
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If the gain of the noninverting gain stage is high (≥ 5), a good approximation for v
n_res
2
is given by Equation 12.
The noise contribution from resistors R
2
, R
4
, R
5
, and R
6
when referred to the input is smaller in comparison to R
1
and R
3
and can be neglected for approximation purposes.
v
n_res
2
= 4 · k · T · (R
1
+ R
3
) · Δf
where
• k = Boltzmann constant
• T = temperature (in kelvins)
• Δf = noise bandwidth (12)
An approximation for the input impedance, R
IN
, of the application circuit is given by Equation 13. R
IN
can be
modeled as a resistor in parallel with the shunt resistor, and can contribute to additional gain error.
R
IN
= R
3
+ R
4
(13)
From Equation 12 and Equation 13, a trade-off exists between v
N
and R
IN
. If R
3
increases, v
n_res
increases, and
therefore, the total input-referred rms system noise, v
N
, increases. If R
3
decreases, the input impedance, R
IN
,
drops, and causes additional gain error.
9.2.2.6 First-order RC Filter Considerations
Although the device digital filter attenuates high-frequency noise, use a first order low-pass RC filter at the ADC
inputs to further reject out-of-bandwidth noise and avoid aliasing. A differential low-pass RC filter formed by R5,
R6, and the differential capacitor C
DIFF
sets the –3-dB cutoff frequency, f
C
, given by Equation 14. These filter
resistors produce a voltage drop because of the input currents flowing into and out of the ADC. This voltage drop
could contribute to an additional gain error. Limit the filter resistor values to below 1 k Ω.
f
C
= 1 / [2π · (R
5
+ R
6
) · C
DIFF
] (14)
Two common-mode filter capacitors (C
CM1
and C
CM2
) are also added to offer attenuation of high-frequency,
common-mode noise components. Select a differential capacitor, C
DIFF
, that is at least an order of magnitude
(10x) larger than these common-mode capacitors because mismatches in these common-mode capacitors can
convert common-mode noise into differential noise.
9.2.2.7 Circuit Implementation
Table 9 shows the chosen values for this design.
(1) 1% precision resistors used
Table 9. Parameters
PARAMETER VALUE
V
CM
2.5 V
FSR of ADC ±0.256 V
Output Data Rate 250 SPS
R
1
, R
3
1 kΩ
(1)
R
2
, R
4
5 kΩ
(1)
R
5
, R
6
100 Ω
(1)
C
DIFF
0.22 µF
C
CM1
, C
CM2
0.022 µF
Using Equation 5, if V
SHUNT
ranges from –50 mV to +50 mV, the application circuit produces a differential voltage
ranging from –0.250 V to +0.250 V across the ADC inputs . The ADC is therefore configured at a FSR of ±0.256
V to maximize the dynamic range of the ADC.
The –3 dB cutoff frequencies of the differential low-pass filter and the common-mode low-pass filters are set at
3.6 kHz and 0.36 kHz, respectively.
R
SHUNT
typically ranges from 0.01 mΩ to 100 mΩ. Therefore, if R
1
= R
3
= 1 kΩ, a good trade-off exists between
the circuit input impedance and input referred resistor noise as explained in the Noise and Input Impedance
Considerations section.
A simple resistor divider followed by a buffer amplifier is used to generate V
CM
of 2.5 V from a 5-V supply.