Texas Instruments | ±480-V isolated voltage-sensing circuit with differential output | Application notes | Texas Instruments ±480-V isolated voltage-sensing circuit with differential output Application notes

Texas Instruments ±480-V isolated voltage-sensing circuit with differential output Application notes
Analog Engineer's Circuit: Amplifiers
SBAA321 – January 2019
±480-V isolated voltage-sensing circuit with
differential output
Design Goals
Voltage Source
VMAX
VMIN
+480V
–480V
ISO224 Input
Voltage
VIN,
MAX
+12V
VIN,
MIN
–12V
ISO224 Output Voltage
VDD2 / 2 Common-Mode
(VOUTP – VOUTN)
VOUT,
MAX
+4V
VOUT,
MIN
–4V
Power Supplies
VDD1
VDD2
4.5V–18V
4.5V–5.5V
Design Description
This circuit performs a ±480-V, isolated, voltage-sensing measurement utilizing the ISO224 isolated
amplifier and a voltage-divider circuit. The voltage-divider circuit reduces the voltage from ±480V to ±12V
which matches the input range of the ISO224. The ISO224 is powered from both a high and low side
power supply. Typically, the high side supply is generated using a floating supply or from the low side
using an isolated transformer or isolated DC/DC converter. The ISO224 can measure single-ended signals
of ±12V with a fixed gain of ⅓V/V and produces an isolated differential output voltage of ±4V with an
output common-mode voltage of VDD2 / 2. The differential output voltage can be scaled as necessary
using an additional operational amplifier such as the TLV6001 as shown in SBOA274 to interface with an
ADC.
Design Notes
1. Verify the systems linear operation for the desired input signal range. This is verified using simulation
in the DC Transfer Characteristics section.
2. Verify that the resistors used in the resistor divider circuit (R1–R5) are capable of dissipating the power
supplied from the voltage source.
3. Verify that the voltage on the input of the ISO224 is less than ±15V as stated in the absolute maximum
ratings table of the data sheet and ensure that less than ±10mA is applied to the input. If the system is
susceptible to transients, consider adding a TVS diode to the input. See the I-V Curve of the Input
Clamp Protection Circuit image in the ISO224 Reinforced isolated amplifier with single-ended input of
±12 V and differential output of ±4 V data sheet for additional details.
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Design Steps
1. Calculate the ratio from the voltage source to the input of the ISO224 for the voltage-divider circuit.
12V ISO224,INPUT
480V
= 0.025
2. The typical input impedance of the ISO224 is 1.25MΩ. This impedance is in parallel with resistor R5
and must be considered when designing the voltage-divider circuit. Choose 1-MΩ resistors for R1, R2,
R3, and R4. Using the ratio from the previous step and the following voltage-divider equation, solve for
the equivalent resistance required for the voltage-divider parallel combination ( || ) of R5 and the
ISO224 input impedance.
R5 || RIN,ISO224
= 0.025
R1 + R 2 + R3 + R 4 + R5 || RIN,ISO224
R5 || RIN,ISO224
4MW + R5 || RIN,ISO224
= 0.025
R5 || RIN,ISO224 = 102564W = REQ
3. Substituting 1.25MΩ for the ISO224 input impedance and using the following equation, solve for R5.
Use the analog engineer's calculator to determine the closest standard value for R5.
R5 × RIN,ISO224
R × 1.25MW
= 5
REQ = 102564W =
R5 + RIN,ISO224 R5 + 1.25MW
102564W (R5 + 1.25MW ) = R5 × 1.25MW
R5 = 111.73 kW; closest standard value = 111 kW
4. Verify that the equivalent resistance is close to the calculated resistance from step 2.
R EQ =
R 5 × RIN,ISO224
R 5 + RIN,ISO224
=
111 k W × 1.25 M W
= 101.947 k W
111 k W + 1.25 M W
5. Verify that the voltage-divider circuit is within a reasonable error tolerance. For the following
calculation, the input resistance of the ISO224 is assumed to be the typical value of 1.25MΩ and this
results in an error of 0.6%. However, it is important to keep in mind that the input resistance will vary
from device to device due to variations in the resistance of the internal clamp protection circuit. If the
same calculation is performed using the minimum input resistance of 1MΩ, the error is 2.5%. If this
error range is unacceptable then either a calibration must be performed or the resistance of the
voltage-divider circuit can be scaled down.
101.947k W
= 0.02485
4.101947M W
| Actual - Calculated |
| 0.02485 - 0.025 |
Error% =
× 100 =
× 100 = 0.6%
Calculated
0.025
6. Calculate the current flowing through the voltage-divider circuit from the voltage source to ensure that
the power dissipation does not exceed the ratings of the resistor. For additional details, please see
Considerations for High Voltage Measurements.
V = IR;
2
V
480 V
=
= 117 m A
R 4 M W + 111k W
±480-V isolated voltage-sensing circuit with differential output
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DC Transfer Characteristics
The following graph shows the simulated output for a ±600-V input. The voltage divider scales the gain by
1/40, and the ISO224 scales the gain by an additional ⅓.
The transfer function shows the system gain is 1/40 from the voltage divider and ⅓ from the ISO224 (that
is, Gain · VIN = VOUT, (1/40)·(⅓)·(480V) = 4V).
AC Transfer Characteristics
The simulated gain is –41.58dB (or 0.008337V/V) which closely matches the expected gain for the voltage
divider and ISO224.
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References
1. Analog Engineer's Circuit Cookbooks
2. SPICE Simulation File SBAC232
3. TI Precision Designs TIDA-00835
4. TI Precision Labs
Design Featured Isolated Op Amp
ISO224B
VDD1
4.5V–18V
VDD2
4.5V–5.5V
Input Voltage range
±12V
Nominal Gain
⅓
VOUT
Differential ±4V on output common-mode of VDD2 / 2
Input Resistance
1.25MΩ (typ)
Small Signal Bandwidth
275kHz
Input Offset Voltage and Drift
±5mV (max), ±15µV/°C (max)
Gain Error and Drift
±0.3% (max), ±35 ppm/°C (max)
Nonlinearity and Drift
0.01% (max), ±0.1 ppm/°C (typ)
Isolation Transient Overvoltage
7kVPEAK
Working Voltage
1.5kVRMS
Common-mode transient immunity, CMTI
55 kV/µs (min)
www.ti.com/product/ISO224
Design Alternate Isolated Op Amp
AMC1311B
VDD1
3V–5.5V
VDD2
3V–5.5V
Input Voltage range
2V
Nominal Gain
1
VOUT
Differential ±2V on output common-mode of 1.44V
Input Resistance
1GΩ (typ)
Small Signal Bandwidth
220kHz
Input Offset Voltage and Drift
±1.5mV (max), ±15µV/°C (max)
Gain Error and Drift
±0.3% (max), ±45 ppm/°C (max)
Nonlinearity and Drift
0.01%, 1 ppm/°C (typ)
Isolation Transient Overvoltage
7kVPEAK
Working Voltage
1.5kVRMS
Common-mode transient immunity, CMTI
75kV/µs (min)
www.ti.com/product/AMC1311
4
±480-V isolated voltage-sensing circuit with differential output
Copyright © 2019, Texas Instruments Incorporated
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