Lastly, some notes about maximizing the high-side current monitor performance:

• All resistor values are critical for accurate gain results. The resistor pairs of [R1 and R3] and [R2 and R4]

must be matched as closely as possible to minimize common-mode mismatch error. Use a 0.1% tolerance,

or better. Often, selecting two adjacent resistors on a reel provides close matching compared to random

selection.

• Although current monitoring is often used for monitoring dc supply currents, ac current can also be monitored.

The –3-dB bandwidth, or upper cutoff frequency, of the circuit of is:

H

GBW

f

Noise Gain

(4)

where

• GBW is the amplifier unity gain bandwidth; 750 kHz for the OPA2186.

• Noise gain is equal to the gain as seen looking into the op-amp noninverting input, as shown in Equation 5.

NG

R2

G

1

R1

(5)

For the OPA2186 circuit in Figure 8-3:

:

:

NG

H

30 k

V

G

1

31

1 k

V

750 kHz

f

24.2 kHz

31

Make sure that the amplifier slew rate is sufficient to support the expected output voltage swing range and

waveform. Also, if a single power supply such as 24 V is used, the ac power source applied to the sense input

voltage cannot drop to less than 0 V.

The OPA2186 output can attain a 0 V output level if a small negative voltage is used to power the V– pin

instead of ground. The LM7705 is a switched capacitor voltage inverter with a regulated, low-noise, –0.23-V

fixed voltage output. Powering the OPA2186 V– pin at this level approximately matches the 300

‑mV linear output

voltage swing lower limit, thus extending the output swing to 0 V, or very near 0 V. This configuration greatly

improves the resolution at low sense current levels.

The LM7705 requires only about 78 μA of quiescent current, but be aware that the specified supply range is 3 V

to 5.25 V. The 3.3-V or 5-V supply used by the ADC could be used as a power source.

For more information about amplifier-based, high-side current monitors, see the TI Analog Engineer’s Circuit

Cookbook: Amplifiers.

www.ti.com

OPA2186

SBOS968 – JUNE 2022

Copyright © 2022 Texas Instruments Incorporated

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