米6体育平台手机版

ZHCSQ83A March 2022 – October 2022 INA851

PRODUCTION DATA

封装选项

机械数据 (封装 | 引脚)

RGT|16
- MPQF119H

散热焊盘机械数据 (封装 | 引脚)

RGT|16
- QFND098S

订购信息

9.2.1.2 Detailed Design Procedure

There are two modes of operation for the circuit shown in Figure 9-3: current input and voltage input. This design requires R₁ >> R₂ >> R₃. Given this relationship, the following equation calculates the current input mode transfer function.

Equation 6.

V_{O U T} = V_{D I F F} \times G = - (I_{I N} \times R_{3})

where

V_OUT represents the differential voltage at the INA851 outputs in current input mode.
V_DIFF represents the differential voltage at the INA851 inputs.
G represents the total gain of the INA851
I_IN is the input current to the PLC.

The following equation shows the transfer function for the voltage input mode:

Equation 7.

V_{O U T} = V_{D I F F} \times G = - (V_{I N} \times (\frac{R_{2}}{R_{2} + R_{1}}))

where

V_OUT represents the differential voltage at the INA851 outputs in voltage input mode.
V_IN is the input voltage to the PLC.

The voltages on the output pins of the INA851 follow the relationships in Equation 8 and Equation 9.

Equation 8.

V_{O U T +} = V_{D I F F} \times \frac{G}{2} + V_{O C M}

Equation 9.

V_{O U T -} = {- V}_{D I F F} \times \frac{G}{2} + V_{O C M}

R₁ sets the input impedance of the voltage input mode. The minimum typical input impedance is 100 kΩ. The R₁ value is 100 kΩ because increasing the R₁ value also increases noise. The value of R₃ must be extremely small compared to R₁ and R₂. A 20‑Ω value is selected for R₃ because that resistance value is much smaller than R₁ and yields an input voltage of ±400 mV when operated in current mode (±20 mA).

Use Equation 10 to calculate R₂ given V_DIFF = ±400 mV, V_IN = ±10 V, and R₁ = 100 kΩ.

Equation 10.

V_{D I F F} = V_{I N} \times \frac{R_{2}}{R_{1} + R_{2}} \to R_{2} = \frac{R_{1} \times V_{D I F F}}{V_{I N} - V_{D I F F}} = 4.167 k Ω

The value obtained from Equation 10 is not a standard 0.1% value; therefore, 4.17 kΩ is selected. R₁ and R₂ also use 0.1% tolerance resistors to minimize error.

Use Equation 11 to calculate the gain of the instrumentation amplifier.

Equation 11.

G = \frac{V_{O U T}}{V_{D I F F}} = \frac{4.95 V}{400 m V} = 12.375 V/V

Equation 12 calculates the gain-setting resistor value using the INA851 gain equation for G_OUT = 1 V/V (Equation 1).

Equation 12.

R_{G} = \frac{6 k Ω}{G - 1} = \frac{6 k Ω}{12.375 - 1} = 527.473 Ω

Use a standard 0.1% resistor value of 530 Ω for this design.

The ADS8920B is selected because of the differential input, 1-MSPS sampling rate, and integrated reference buffer. Implement the antialiasing R-C-R filter using two 47.4-Ω resistors, a COG or NPO-type 510-pF differential capacitor, and two ceramic 51-pF common-mode capacitors. The REF5050 is selected to create a 5-V reference voltage for the ADC. Use well-matched precision resistors to create a voltage divider that generates a stable 2.5-V V_OCM reference. Connect the VCLAMP+ and VCLAMP− pins of the INA851 to the supplies of the ADC to protect against overdrive damage in the event of a fault. Consider implementing a TVS diode from the ADC supply to GND for additional protection, and include 100-nF decoupling capacitors between the amplifier and ADC supplies and GND.