Dual, 65msps, 14-bit, if/baseband adc, Grounding, bypassing, and board layout – Maxim Integrated MAX12557 User Manual
Page 24
MAX12557
ence, allowing REF_P, REF_N, and COM_ to be driven
directly by a set of external reference sources.
Figure 13 uses a MAX6029 precision 3.000V bandgap
reference as a common reference for multiple convert-
ers. A seven-component resistive divider chain follows
the MAX6029 voltage reference. The 0.47µF capacitor
along this chain creates a 10Hz LP filter. Three
MAX4230 amplifiers buffer taps along this resistor
chain providing 2.413V, 1.647V, and 0.880V to the
MAX12557 REF_P, REF_N, and COM_ reference
inputs. The feedback around the MAX4230 op amps
provides additional 10Hz LP filtering. Reference volt-
ages 2.413V and 0.880V set the full-scale analog input
range for the converter to ±1.022V (±[V
REF_P
- V
REF_N
]
x 2/3).
Note that one single power supply for all active circuit
components removes any concern regarding power-
supply sequencing when powering up or down.
Grounding, Bypassing, and
Board Layout
The MAX12557 requires high-speed board layout
design techniques. Refer to the MAX12557 EV kit data
sheet for a board layout reference. Locate all bypass
capacitors as close to the device as possible, prefer-
ably on the same side as the ADC, using surface-
Dual, 65Msps, 14-Bit, IF/Baseband ADC
24
______________________________________________________________________________________
MAX12557
MAX4230
MAX6029
(EUK30)
0.1
µF
1
5
2
0.47
µF
10
µF
6V
47
Ω
1.47k
Ω
2.413V
3V
4
1
3
330
µF
6V
MAX4230
10
µF
6V
47
Ω
1.47k
Ω
1.647V
4
1
3
330
µF
6V
MAX4230
10
µF
6V
47
Ω
1.47k
Ω
0.880V
4
1
3
330
µF
6V
REF_P
REF_N
COM_
V
DD
GND
REFIN
3.3V
3.3V
REFOUT
0.1
µF
0.1
µF
0.1
µF
10
µF
0.1
µF
2.2
µF
0.1
µF
20k
Ω
1%
20k
Ω
1%
20k
Ω
1%
20k
Ω
1%
20k
Ω
1%
52.3k
Ω
1%
52.3k
Ω
1%
0.1
µF
MAX12557
REF_P
REF_N
COM_
V
DD
GND
REFIN
REFOUT
0.1
µF
0.1
µF
0.1
µF
10
µF
0.1
µF
2.2
µF
0.1
µF
0.1
µF
Figure 13. External Unbuffered Reference Driving Multiple ADCs