OPA862_V02 datasheet(19/37 Pages) TI

номер детали	OPA862

подробное описание детали	OPA862 High Input Impedance, Single-Ended to Differential ADC Driver

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7.3.2 Anti-Phase Reversal Protection

When the input common-mode voltage approaches or exceeds VS–, the base-collector junction of the input

transistors forward biases. This condition creates an output path parallel to the normal gm path of the transistors

that is opposite in phase to the gm path. When this parallel path starts to dominate, phase inversion occurs. To

protect against phase inversion, the OPA862 features anti-phase reversal (APR) protection Schottky diodes on

the input transistors. The Schottky diodes turn on at a voltage lower than the forward bias voltage of the base-

collector junction, thus preventing the forward bias and the phase-inversion at the base-collector junction of the

input transistors. Figure 7-2 shows a diagram of APR protection within the OPA862.

Internal Circuitry

VFB

VIN

VS+

Figure 7-2. Anti-Phase Reversal Protection

7.3.3 Precision and Low Noise

The OPA862 is laser trimmed for high DC precision. An important factor that reduces the DC precision of the

system that uses the OPA862 is the errors introduced by the bias currents of A2 flowing through the internal

feedback resistors, RINT; see Section 7.2. To minimize the error contribution from IB, the A2 amplifier of the

OPA862 features a unique IB cancellation mechanism. This IB cancellation mechanism is the reason why the IB

of A2 is orders of magnitude lower than the IB of A1. The DC errors are negligible for most applications because

of the nanoamperes of IB and very low IB drift of A2. However, despite being very low, if the IB errors of A2 are

significant for an application, a 348-Ω RREF resistor can be used on the VREF input to cancel out the IB errors.

The tradeoff of using the RREF is that this resistor introduces noise that is amplified by a factor of two at VOUT–

because of the noise gain of two of A2. The CFILT capacitor (see Section 7.2) also helps filter out the flat band

noise contribution of RREF. The 700-Ω internal resistors were carefully chosen to balance low noise while

keeping the total power dissipation low by taking advantage of the low 3.1-mA quiescent current of the OPA862.

As shown in Figure 7-3, to get the equivalent 8.3-nV/√ Hz noise of the OPA862 with a typical FDA configuration,

the FDA must be less than 1 nV/√ Hz; such FDAs are often difficult to find or expensive. When RREF equals 0 Ω,

the typical error resulting from the IB of A2 appears as an input-referred offset of 3.5 µV at the VREF input, and

when RREF is 348 Ω, the differential output-referred noise increases from 8.3 nV/√ Hz to 9.6 nV/√ Hz.

VOUT+

VOUT±

A1

+

±

A2

+

±

700

1

8

5

OPA862

RINT

4

2

+

±

Fully Differential Amplifier

VOCM

VOD

1 k

G = 2 V/V

G = 1 V/V

IB noise ignored

VOD

8.3 nV/¥+]

<1 nV/¥+]

Figure 7-3. Equivalent Voltage Noise FDA to OPA862

www.ti.com

OPA862

SBOS919C – AUGUST 2019 – REVISED AUGUST 2020

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