"Chapter 1 - The Op Amp's Place in the World" - HTL Wien 10

THS5672
DAC
IOUT1
IOUT2
C fb
3.1827 104
f clk
2
49.9 Ω
499 Ω
R 1
R 1
V COM,OUT
C fb
R 2
R 2
499 Ω
49.9 Ω 499 Ω
Figure 13–7. A Single-Pole Reconstruction Filter
13.7 External V ref Circuits for ADCs/DACs
499 Ω
C fb
External Vref Circuits for ADCs/DACs
THS4131
THS4141
THS4151
Wireless Communication: Signal Conditioning for IF Sampling
+)
– )
∆VOUT
(13–5)
Figure 13–8 shows an op amp voltage follower circuit that is often used to interface the
external precision voltage reference supplying the ADC/DAC external reference voltage
(see for example, Miller and Moore, [5], [6],1999, 2000 for a more detailed discussion on
voltage reference circuits used in ADC an DAC systems). V in is the output from a precision
voltage reference, such as the Thaler Corp. VRE3050. The low-pass filter (formed by
C 1R 1) filters noise from the reference and op amp buffer. The –3 dB corner frequency of
the filter is 1/2πC 1R 1and the transfer function for this circuit can be written as
Vo
Vin s 2 sC 2 R 2
C 1 C 2 R 1 R 2
which has a zero at s = C 2R 2.
1 sC 2 R 2
1
C C1C2R1R 2 1C2R1R2 (13–6)
With the approximation C 2R 2 = 2C 1R 1, the denominator polynomial is solved for complex
poles p 1 and p 2 of the response, which results in:
13-15