Datasheet

-
+
R
S
R
L
2 k:
V
OUT
LMH6601
U1
R
G
R
F
C
F
C
L
V
IN
+
-
LMH6601
U1
R
F
C
F
V
OUT
D
1
10 k:
R
S
C
D
V
BIAS
1000 pF
R
F
= 10 M: to 10 G:
R
S
= 1 M: or SMALLER FOR HIGH COUNTING RATES
C
F
= 1 pF
C
D
= 1 pF to 10 PF
V
OUT
= Q/C
F
WHERE Q is CHARGE
CREATED BY ONE PHOTON or PARTICLE
ADJUST V
BIAS
FOR MAXIMUM SNR
+
-
LMH6601, LMH6601-Q1
www.ti.com SNOSAK9E –JUNE 2006–REVISED MARCH 2013
OTHER APPLICATIONS
Figure 62. Charge Preamplifier Taking Advantage of LMH6601’s Femto-Ampere Range Input Bias Current
CAPACITIVE LOAD
The LMH6601 can drive a capacitive load of up to 1000 pF with correct isolation and compensation. Figure 63
illustrates the in-loop compensation technique to drive a large capacitive load.
Figure 63. In-Loop Compensation Circuit for Driving a Heavy Capacitive Load
When driving a high capacitive load, an isolation resistor (R
S
) should be connected in series between the op amp
output and the capacitive load to provide isolation and to avoid oscillations. A small value capacitor (C
F
) is
inserted between the op amp output and the inverting input as shown such that this capacitor becomes the
dominant feedback path at higher frequency. Together these components allow heavy capacitive loading while
keeping the loop stable.
There are few factors which affect the driving capability of the op amp:
• Op amp internal architecture
• Closed loop gain and output capacitor loading
Table 4 shows the measured step response for various values of load capacitors (C
L
), series resistor (R
S
) and
feedback resistor (C
F
) with gain of +2 (R
F
= R
G
= 604Ω) and R
L
= 2 kΩ:
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