SBOU252 User guide

SBOU252 August 2021 OPA4991 , OPA4991-Q1 , TLV9004 , TLV9004-Q1

4.2 Difference Amplifier

Figure 4-3 displays the schematic for the difference amplifier circuit configuration.

Figure 4-3 Difference Amplifier Schematic

Table 4-2 Difference Amplifier Components

CH	R_INM	R_F	C_F	R_INP	R_VREF	C_VREF	R_PL	R_Iso	C_L
1	R1	R2	C3	R3	R4	C4	R6	R5	C5
2	R7	R8	C6	R9	R10	C7	R12	R11	C8
3	R13	R14	C9	R15	R16	C10	R18	R17	C11
4	R19	R20	C12	R21	R22	C13	R24	R23	C14

Equation 1 displays the transfer function of the difference amplifier circuit configuration. Note, Input signals IN+ and IN- are altered to IN_P and IN_M respectively in the transfer function for simplicity of the equation.

Equation 1.

OUT = (\frac{R_{VREF}}{R_{INP} + R_{VREF}}) \times (1 + \frac{R_{F}}{R_{INM}}) \times {IN}_{P} + (\frac{R_{INP}}{R_{INP} + R_{VREF}}) \times (1 + \frac{R_{F}}{R_{INM}}) \times Vref - \frac{R_{F}}{R_{INM}} \times {IN}_{M}

If R_F = R_VREF and R_INM = R_INP, Equation 1 can simplify to Equation 2.

Equation 2.

OUT = \frac{R_{F}}{R_{INM}} \times ({IN}_{P} - {IN}_{M}) + Vref

Capacitors C_F and C_VREF provide the option to filter the output of the amplifier. Equation 3 calculates the cutoff frequency, f_c, of the filter; where R_F = R_VREF, R_INM = R_INP, and C_F = C_VREF.

Equation 3.

f_{c} = \frac{1}{2 \times π \times R_{F} \times C_{F}}

Resistor R_PL can be used to set the Vref pin to GND. A grounded Vref is best used with a split power supply configuration. Do not externally drive the Vref pin if R_PL is populated on the PCB.

Resistor R_iso, and capacitor C_L provide the option to create a RC filter, or test output loads for the amplifier. When not applicable, use a zero ohm resistor for R_iso and do not populate C_L.

Figure 4-4 displays the DYY-AMP-EVM populated with the required components to configure channel 1 as a difference amplifier with no load and VREF tied to ground.

Figure 4-4 Difference Amplifier Configured on DYY-AMP-EVM, Channel 1

CH	R_INM	R_F	C_F	R_INP	R_VREF	C_VREF	R_PL	R_Iso	C_L
1	R1	R2	C3	R3	R4	C4	R6	R5	C5
2	R7	R8	C6	R9	R10	C7	R12	R11	C8
3	R13	R14	C9	R15	R16	C10	R18	R17	C11
4	R19	R20	C12	R21	R22	C13	R24	R23	C14

CH	R_INM	R_F	C_F	R_INP	R_VREF	C_VREF	R_PL	R_Iso	C_L
1	R1	R2	C3	R3	R4	C4	R6	R5	C5
2	R7	R8	C6	R9	R10	C7	R12	R11	C8
3	R13	R14	C9	R15	R16	C10	R18	R17	C11
4	R19	R20	C12	R21	R22	C13	R24	R23	C14

CH	R_INM	R_F	C_F	R_INP	R_VREF	C_VREF	R_PL	R_Iso	C_L
1	R1	R2	C3	R3	R4	C4	R6	R5	C5
2	R7	R8	C6	R9	R10	C7	R12	R11	C8
3	R13	R14	C9	R15	R16	C10	R18	R17	C11
4	R19	R20	C12	R21	R22	C13	R24	R23	C14