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TS1002IM8T Scheda tecnica(PDF) 9 Page - Silicon Laboratories |
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TS1002IM8T Scheda tecnica(HTML) 9 Page - Silicon Laboratories |
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9 / 14 page  TS1002/TS1004 TS1002/4 Rev. 1.0 Page 9 The circuit utilizes the classic two op amp instrumentation amplifier topology with four resistors to set the gain. The equation is simply that of a noninverting amplifier as shown in the figure. The two resistors labeled R1 should be closely matched to each other as well as both resistors labeled R2 to ensure acceptable common-mode rejection performance. Resistor networks ensure the closest matching as well as matched drifts for good temperature stability. Capacitor C1 is included to limit the bandwidth and, therefore, the noise in sensitive applications. The value of this capacitor should be adjusted depending on the desired closed-loop bandwidth of the instrumentation amplifier. The RC combination creates a pole at a frequency equal to 1/(2 π × R1C1). If the AC-CMRR is critical, then a matched capacitor to C1 should be included across the second resistor labeled R1. Because these amplifiers accept rail-to-rail inputs, their input common mode range includes both ground and the positive supply of 1.5V. Furthermore, their rail-to-rail output range ensures the widest signal range possible and maximizes the dynamic range of the system. Also, with their low supply current of 0.6 μA per amplifier, this circuit consumes a quiescent current of only ~1.3 μA, yet it still exhibits a 1-kHz bandwidth at a circuit gain of 2. Driving Capacitive Loads While the amplifiers’ internal gain-bandwidth product is 4kHz, both are capable of driving capacitive loads up to 50pF in voltage follower configurations without any additional components. In many applications, however, an operational amplifier is required to drive much larger capacitive loads. The amplifier’s output impedance and a large capacitive load create additional phase lag that further reduces the amplifier’s phase margin. If enough phase delay is introduced, the amplifier’s phase margin is reduced. The effect is quite evident when the transient response is observed as there will appear noticeable peaking/ringing in the output transient response. If any amplifier is used in an application that requires driving larger capacitive loads, an isolation resistor between the output and the capacitive load should be used as illustrated in Figure 5. Table 1 illustrates a range of RISO values as a function of the external CLOAD on the output of these amplifiers. The power supply voltage applied on the these amplifiers at which these resistor values were determined empirically was 1.8V. The oscilloscope capture shown in Figure 6 illustrates a typical transient response obtained with a CLOAD = 500pF and an RISO = 50k Ω. Note that as CLOAD is increased a smaller RISO is needed for optimal transient response. External Capacitive Load, CLOAD External Output Isolation Resistor, RISO 0-50pF Not Required 100pF 120k Ω 500pF 50k Ω 1nF 33k Ω 5nF 18k Ω 10nF 13k Ω Figure 5: Using an External Resistor to Isolate a CLOAD from the Amplifer’s Output. Figure 4: A Two Op Amp Instrumentation Amplifier. |
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