Datasheet

LTC2378-20
11
237820f
For more information www.linear.com/LTC2378-20
applicaTions inForMaTion
The ADC inputs may be modeled as a switched capacitor
load of the drive circuit. A drive circuit may rely partially
on attenuating switched-capacitor current spikes with
small filter capacitors C
FILT
placed directly at the ADC
inputs, and partially on the driver amplifier having suffi-
cient bandwidth to recover from the residual disturbance.
Amplifiers optimized for DC performance may not have
sufficient bandwidth to fully recover at the ADC’s maximum
conversion rate, which can produce nonlinearity and other
errors. Coupling filter circuits may be classified in three
broad categories:
Fully Settled – This case is characterized by filter time
constants and an overall settling time that is consider-
ably shorter than the sample period. When acquisition
begins, the coupling filter is disturbed. For a typical first
order RC filter, the disturbance will look like an initial step
with an exponential decay. The amplifier will have its own
response to the disturbance, which may include ringing. If
the input settles completely (to within the accuracy of the
LTC2378-20), the disturbance will not contribute any error.
Partially Settled – In this case, the beginning of acquisition
causes a disturbance of the coupling filter, which then
begins to settle
out towards the nominal input voltage.
However,
acquisition ends (and the conversion begins)
before the input settles to its final value. This generally
produces a gain error, but as long as the settling is linear,
no distortion is produced. The coupling filter’s response
is affected by the amplifier’s output impedance and other
parameters. A linear settling response to fast switched-
capacitor current spikes can NOT always be assumed for
precision, low bandwidth amplifiers. The coupling filter
serves to attenuate the current spikes’ high-frequency
energy before it reaches the amplifier.
Fully Averaged – If the coupling filter capacitors (C
FILT
) at the
ADC inputs are much larger than the ADC’s sample capacitors
(45pF), then the sampling glitch is greatly attenuated. The
driving amplifier effectively only sees the average sampling
current, which is quite small. At 1Msps, the equivalent input
resistance is approximately 22k (as shown in Figure 5), a
benign resistive load for most precision amplifiers. However,
resistive voltage division will occur between the coupling
filter’s DC resistance and the ADC’s equivalent (switched-
capacitor) input resistance, thus producing a gain error.
The input leakage currents of the LTC2378-20 should
also be considered when designing the input drive
circuit,
because
source impedances will convert input leakage
currents to an added input voltage error. The input leakage
currents, both common mode and differential, are typically
extremely small over the entire operating temperature
range. Figure 6 shows input leakage currents over tem-
perature for a typical part.
Let R
S1
and R
S2
be the source impedances of the dif-
ferential input drive circuit shown in Figure 7, and let I
L1
and I
L2
be the leakage currents flowing out of the ADC’s
analog inputs. The voltage error, V
E
, due to the leakage
currents can be expressed as:
V
E
=
R
S1
+
R
S2
2
• I
L1
–I
L2
( )
+ R
S1
–R
S2
( )
•
I
L1
+I
L2
2
The common mode input leakage current, (I
L1
+ I
L2
)/2, is
typically extremely small (Figure 6) over the entire operat-
Figure 5. Equivalent Circuit for the Differential Analog
Input of the LTC2378-20 at 1Msps
Figure 6. Common Mode and Differential Input Leakage Current
over Temperature
LTC2378-20
BIAS
VOLTAGE
IN
+
IN
–
C
FILT >> 45pF
237820 F05
R
EQ
R
EQ
C
FILT >> 45pF
R
EQ
=
1
f
SMPL
• 45pF
TEMPERATURE (°C)
INPUT LEAKAGE (nA)
30
237820 F06
–10
0
10
20
–55 –35 –15 5 25 6545 85
DIFFERENTIAL
COMMON