SBOA575 Application note

SBOA575 September 2023 INA823 , OPA2387 , XTR115 , XTR116

2.2 2-Amp INA

A discrete 2-amp INA is selected to amplify the differential voltage developed by the bridge. A discrete solution is selected over an integrated INA because a wide gain range and zero-drift technology for high precision over temperature is required. The circuit configuration and transfer function are shown below in Figure 2-4 and Equation 4 respectively.

Figure 2-4 Discrete 2-Amp INA Configuration

Equation 4.

V_{o u t_I N A} = (1 + \frac{R 3}{R 2} + \frac{2 R 2}{R G}) * (V 2 - V 1) + V_{R E F}, w h e r e R 1 = R 2 a n d R 3 = R 4

Experimentation with the load cell showed a change in V_DIFF of approximately 95-μV/lb. Hence, a gain stage is necessary to increase the output voltage to an appropriate range to feed into the 4-20-mA transmitter. The gain of the 2-amp INA is calculated below. The desired output voltage of the INA is 0.5-V at 0-lbs and 4.5-V at 20-lbs.

Equation 5.

V_{D I F F_M A X} = s e n s i t i v i t y \times \max l o a d = 95 μ V / l b \times 20 l b s \to V_{D I F F_M A X} = 1.9 m V

Equation 6.

G a i n = \frac{(V_{o u t m a x_I N A} - V_{o u t m i n_I N A})}{V_{D I F F_M A X}} = \frac{4.5 V - 0.5 V}{1.9 m V} \to G a i n = 2105 V / V

The required gain is used to calculate the resistor values of the 2-amp INA. R1 and R2 are set to 100 kΩ and R3 and R4 are set to 10 kΩ.

Equation 7.

R G = \frac{2 * R 2}{G a i n - 1 - \frac{R 3}{R 2}} = \frac{2 * 100 k Ω}{2105 V / V - 1 - \frac{100 k Ω}{10 k Ω}} = 95.511 Ω \to R G = 95.3 Ω (s t a n d a r d v a l u e)

Equation 8.

G a i n = (1 + \frac{R 3}{R 2} + \frac{2 R 2}{R G}), w h e r e R 1 = R 2 a n d R 3 = R 4

DC precision and noise are key considerations when choosing an amplifier for bridge sensing. Low input offset voltage, drift and input bias current are vital to achieve a high accuracy output. An amplifier with low 1/f noise is important because bridge sensors are typically used with low signal frequencies. The OPA2387 with Zero-Drift Amplifiers: Features and Benefits, application brief is selected for the 2-amp INA. Table 2-1 summarizes the key specifications for the bridge amplifier stage and suggests other precision amplifiers that are designed for this application.

Table 2-1 Key Specifications for Bridge Amplifier Device

Device	OPAx387	OPAx333	OPAx186
Supply (V)	1.7 to 5.5	1.8 to 5.5	4.5 to 24
Vos (max, μV)	2	10	5
Vos drift (typ, nV/°C)	3	20	1
Input bias current (typ, pA)	30	70	100
Noise (0.1 to 10 Hz, nV/√Hz)	27	170	125
Iq per channel (typ, μA)	570	17	90

The output voltage of the 2-amp INA is approximately 0.345-V when no weight is applied to the load cell. Therefore, a reference of 0.155-V is implemented to boost the output voltage up to 0.5-V at 0-lbs. TI’s Analog Engineer’s Calculator can be used to calculate the standard resistor values that offer the lowest possible reference voltage error.

The reference voltage of the INA (V_{REF INA}) is set using a voltage divider between the 4.096-V XTR reference and the local ground where the series resistance must be in the 10’s of kΩ to limit current consumption. V_{REF INA} must be driven with a low-impedance source to prevent a voltage drop due to the input resistor, R1. The TLV333 was selected to buffer the reference voltage due to its low power consumption and high DC precision.

Equation 9.

V_{R E F_I N A} = 0.155 V = V_{R E F_X T R} \times \frac{R_{B 2}}{R_{B 1} + R_{B 2}} \to R_{B 1} = 76.8 k Ω, R_{B 2} = 3.01 k Ω