Page 115 - Mechanical Engineers' Handbook (Volume 2)

P. 115

104 Bridge Transducers

8. Frequency Response. The minimum frequency range over which the ampliﬁer gain
is within 3 dB of the dc level for all speciﬁed gains for any output signal am-
plitude within the linear output voltage range. In writing speciﬁcations, it is not
uncommon for a user to also specify the desired phase characteristics over the
frequency range of interest and the number of ﬁlter poles.
9. Slew Rate. The maximum rate at which the ampliﬁer can change output voltage
from the minimum to the maximum limit of linear output voltage range. It is ex-
pressed in volts per microsecond with a large-amplitude step voltage applied to the
input of the ampliﬁer and the ampliﬁer driving a speciﬁed capacitive load. The
usual source of slew rate difﬁculty is current limiting, and this speciﬁcation (a
nonlinear process) should not be confused with rise time (a linear process).
10. Settling Time. The time following the application of a step voltage input for the
ampliﬁer output voltage to settle to within a speciﬁed percentage of its ﬁnal value.
11. Overload Recovery. The time required for the ampliﬁer to recover from a speciﬁed
differential input signal overload. It is speciﬁed as the number of microseconds
from the end of the input overload to the time that the ampliﬁer dc output voltage
recovers to within the linear output voltage range. Ampliﬁer gain must be speciﬁed.
12. Noise. Noise is divided into two components: RTI and RTO. RTI noise is that
component of noise that varies directly with gain. It is measured with the ampliﬁer
input leads terminated in the maximum source impedance and no signal applied.
The RTO noise is that component of noise which remains ﬁxed with gain.
13. Harmonic Distortion. The maximum harmonic content for any ampliﬁer frequency
or output amplitude within the speciﬁed limits.
14. Output Impedance. The maximum impedance the ampliﬁer will present when it is
operated anywhere within its speciﬁcation. This speciﬁcation is important in resis-
tive loading ratings or in determining the amount of capacitance which can be
connected across the output without causing instability.
Reference 10 describes test procedures for these speciﬁcations and discusses them fur-
ther. Figure 27 presents the basic dc ampliﬁer circuit. Reference 11 provides additional
discussion directed toward understanding dc instrumentation ampliﬁers.

7.3 Slip Rings

In many measurement applications, it is necessary to acquire data from rotating machinery.
Turbines, rate tables, and centrifuges are examples of such machinery. If it is necessary to

Figure 27 Basic dc ampliﬁer circuit.

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