OPA835IDBVT datasheet(32/63 Pages) TI2

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OPA835

VIN+

VIN-

VCM

VREF

VSIG-

VSIG+

G[(V

)-(

SIG+

)]

SIG-

VOUT

VREF

Figure 9-4. Differential to Single-Ended Amplifier

Line termination can be accomplished by adding a shunt resistor across the VIN+ and VIN- inputs. The

differential impedance is the shunt resistance in parallel with the input impedance of the amplifier circuit, which

is usually much higher. For low gain and low line impedance, the resistor value to add is approximately the

impedance of the line. For example, if a 100-Ω Cat5 cable is used with a gain of 1 amplifier and RF = RG = 2

kΩ, adding a 100-Ω shunt across the input will give a differential impedance of 99 Ω, which is adequate for most

applications.

For best CMRR performance, resistors must be matched. Assuming CMRR ≈ the resistor tolerance, a 0.1%

tolerance will provide about 60-dB CMRR.

9.1.7 Differential-to-Differential Amplifier

Figure 9-5 shows a differential amplifier that is used to amplify differential signals. This circuit has high input

impedance and is used in differential line driver applications where the signal source is a high-impedance driver

(for example, a differential DAC) that must drive a line.

If the user sets VIN± = VCM + VSIG±, then the output of the amplifier may be calculated according to Equation 6.

´ ç

= V

1 +

+ V

OUT

(6)

The signal gain of the circuit is

G = 1 +

, and VCM passes with unity gain. The amplifier combines two

noninverting amplifiers into one differential amplifier that shares the RG resistor, which makes RG effectively ½ its

value when calculating the gain. The output signals are in-phase with the input signals.

SIG-

IN-

SIG-

SIG+

IN+

OUT-

OUT+

SIG+

OPA2835

Figure 9-5. Differential-to-Differential Amplifier

OPA835, OPA2835

SLOS713J – JANUARY 2011 – REVISED MARCH 2021

www.ti.com

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