2001-2012 Microchip Technology Inc. DS21464C-page 7
TC7652
4.3 Output Stage/Load Driving
The output circuit is high impedance (about 18k).
With lesser loads, the chopper amplifier behaves
somewhat like a transconductance amplifier with an
open-loop gain proportional to load resistance. (For
example, the open-loop gain is 17dB lower with a 1k.
load than with a 10k load.) If the amp is used only for
DC, the DC gain is typically greater than 120dB (even
with a 1k load), and this lower gain is inconsequential.
For wide band, the best frequency response occurs
with a load resistor of at least 10k. This produces a
6dB/octave response from 0.1Hz to 2MHz, with phase
shifts of less than 2 degrees in the transition region,
where the main amplifier takes over from the null ampli-
fier.
FIGURE 4-1: CONNECTION OF INPUT GUARDS
4.4 Thermoelectric Effects
The thermoelectric (Seebeck) effects in thermocouple
junctions of dissimilar metals, alloys, silicon, etc. limit
ultra high precision DC amplifiers. Unless all junctions
are at the same temperature, thermoelectric voltages
around 0.1V/C (up to tens of V/C for some materi-
als) are generated. To realize the low offset voltages of
the chopper, avoid temperature gradients. Enclose
components to eliminate air movement, especially from
power dissipating elements in the system. Where pos-
sible, use low thermoelectric co-efficient connections.
Keep power supply voltages and power dissipation to a
minimum. Use high impedance loads and seek maxi-
mum separation from surrounding heat disipating ele-
ments.
4.5 Guarding
To benefit from TC7652 low input currents, take care
assembling printed circuit boards. Clean boards with
alcohol or TCE and blow dry with compressed air. To
prevent contamination, coat boards with epoxy or sili-
cone rubber.
Even if boards are cleaned and coated, leakage cur-
rents may occur because input pins are next to pins at
supply potentials. To reduce this leakage, use guarding
to lower the voltage difference between the inputs and
adjacent metal runs. The guard (a conductive ring sur-
rounding inputs) is connected to a low impedance point
at about the same voltage as inputs. The guard
absorbs leakage currents from high voltage pins.
The 14-pin dual-in-line arrangement simplifies guard-
ing. Like the LM108 pin configuration (but unlike the
101A and 741), pins next to inputs are not used.
Input
+
Output
R
2
R
1
Inverting Amplifier
Input
+
Output
Follower
Input
+
Output
R
2
R
1
Noninverting Amplifier
TC7652
TC7652
TC7652
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