Datasheet

LMH6618
ADC121S101
0.1 PF
C
6
10 PF
C
5
0.1 PF
C
13
10 PF
C
11
0.1 PF
V
+
0.01 PF
14.3 k:
0.1 PF
1 PF
GND
5V
1.24 k:
1 nF
150 pF
549:
V
+
-
+
22:
390 pF
549:
IN
1 PF
V
+
14.3 k:
5.6 PF
D1
D2
D3
D4
NON-INVERTING
INPUT
INVERTING
INPUT
R
S
50:
LMH6618, LMH6619
SNOSAV7E –AUGUST 2007–REVISED OCTOBER 2012 www.ti.com
Figure 63. Input Equivalent Circuit During Shutdown
When the LMH6618 is shutdown, there may be current flow through the internal diodes shown, caused by input
potential, if present. This current may flow through the external feedback resistor and result in an apparent output
signal. In most shutdown applications the presence of this output is inconsequential. However, if the output is
“forced” by another device, the other device will need to conduct the current described in order to maintain the
output potential.
To keep the output at or near ground during shutdown when there is no other device to hold the output low, a
switch using a transistor can be used to shunt the output to ground.
SINGLE CHANNEL ADC DRIVER
The low noise and wide bandwidth make the LMH6618 an excellent choice for driving a 12-bit ADC. Figure 64
shows the schematic of the LMH6618 driving an ADC121S101. The ADC121S101 is a single channel 12-bit
ADC. The LMH6618 is set up in a 2nd order multiple-feedback configuration with a gain of −1. The −3 dB point is
at 500 kHz and the −0.01 dB point is at 100 kHz. The 22Ω resistor and 390 pF capacitor form an antialiasing
filter for the ADC121S101. The capacitor also stores and delivers charge to the switched capacitor input of the
ADC. The capacitive load on the LMH6618 created by the 390 pF capacitor is decreased by the 22Ω resistor.
Table 1 shows the performance data of the LMH6618 and the ADC121S101.
Figure 64. LMH6618 Driving an ADC121S101
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