OPA348-Q1 Datasheet, PDF (16/30 Pages) Texas Instruments – OPAx348-Q1 1-MHz 45-μA CMOS Rail-to-Rail Operational Amplifier

OPA348-Q1, OPA2348-Q1, OPA4348-Q1
SBOS465B – JANUARY 2009 – REVISED DECEMBER 2014
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Typical Application (continued)
8.2.1 Design Requirements
The design requirements are as follows:
• Supply voltage: 2.7 V
• Reference voltage: 2.5 V
• Input: 0.1 V to 2.4 V
• Output differential: ±2.3 V
• Output common-mode voltage: 1.25 V
• Small-signal bandwidth: 1 MHz
8.2.2 Detailed Design Procedure
The circuit in Figure 28 takes a single-ended input signal, V_IN, and generates two output signals, V_OUT+ and
V_OUT– using two amplifiers and a reference voltage, V_REF. V_OUT+ is the output of the first amplifier and is a
buffered version of the input signal, V_IN (as shown in Equation 1). V_OUT– is the output of the second amplifier
which uses V_REF to add an offset voltage to V_IN and feedback to add inverting gain. The transfer function for
V_OUT– is given in Equation 2.
V_OUT V_IN
(1)
V_287±
V₅₍₎
u
§
¨
©
R₄
R₃  R₄
·
¸
¹
u
¨§^1
©
R₂
R₁
·
¸

¹
V_,1
u ^R2
R₁
(2)
The differential output signal, V_DIFF, is the difference between the two single-ended output signals, V_OUT+ and
V_OUT–. Equation 3 shows the transfer function for V_DIFF. By applying the conditions that R₁ = R₂ and R₃ = R₄, the
transfer function is simplified into Equation 6. Using this configuration, the maximum input signal is equal to the
reference voltage and the maximum output of each amplifier is equal to V_REF. The differential output range is 2 ×
V_REF. Furthermore, the common-mode voltage (V_CM) is one half of V_REF (see Equation 7).
V_',))
V_287  V_287±
V_,1
u
§¨_1
©
R₂
R₁
·
¸
¹

V₅₍₎
u
§
¨
©
R₄
R₃  R₄
·
¸
¹
u
¨§^1
R₂
·
¸
© ^R1 ¹
(3)
V_OUT V_IN
(4)
V_287±  V₅₍₎ V_,1
(5)
V_DIFF  2 K V_IN V_REF
(6)
V_CM
§
¨©
V_287

2
V_{287 ±}
·
¸¹
1
2
V_REF
(7)
8.2.2.1 Amplifier Selection
Linearity over the input range is key for good dc accuracy. The common-mode input range and output swing
limitations determine the linearity. In general, an amplifier with rail-to-rail input and output swing is required.
Bandwidth is a key concern for this design, so the OPAx348-Q1 family of devices is selected because its
bandwidth is greater than the target of 1 MHz. The bandwidth and power ratio makes this device power efficient
and the low offset and drift ensure good accuracy for moderate precision applications.
8.2.2.2 Passive Component Selection
Because the transfer function of Vout– is heavily reliant on resistors (R₁, R₂, R₃, and R₄), use resistors with low
tolerances to maximize performance and minimize error. This design uses resistors with resistance values of
49.9 kΩ and tolerances of 0.1%. However, if the noise of the system is a key parameter, smaller resistance
values (6 kΩ or lower) can be selected to keep the overall system noise low. This ensures that the noise from the
resistors is lower than the amplifier noise.
16
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