OPA857_15 Datasheet, PDF (15/30 Pages) Texas Instruments – OPA857 Ultralow-Noise, Wideband, Selectable-Feedback Resistance Transimpedance Amplifier

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OPA857
SBOS630C – DECEMBER 2013 – REVISED APRIL 2014
Feature Description (continued)
When the internal V-to-I converter must be operated, set Test_SD to logic 1, turn on the internal current source,
and set a voltage on Test_IN.
Set an adequate dc voltage at the input to make sure that the output is operating within normal operation. At
minimum, the output of the TIA block must be set to 5/9th of the supply voltage in preparation for a pulse
configuration. For sine-wave operation, as required when measuring a frequency response, set the dc voltage on
the OUT pin to allow the full sine-wave amplitude and avoid clipping. In such a case, the OUT pin voltage is set
lower than 5/9th of the supply voltage.
Note that the 2-kΩ internal resistance used for the V-to-I conversion is not trimmed and can vary ±15% with
process. Therefore, the source must be capable of sourcing both dc and ac voltages to make sure that the output
voltage swing is compliant with the class-A output stage of the TIA block. Any change in the test circuit
configuration (such as gain change) requires a new calibration of the internal V-to-I converter.
Again if a photodiode is used, the internal V-to-I converter must be shut off completely. Failure to do so results in
degraded performance and higher than normal quiescent current.
7.3.4 Internal Clamping Circuit (CLAMP) Block
The OPA857 is built using a very high-speed, complementary, BICMOS process. The internal junction
breakdown voltages are relatively low for these very small geometry devices. These breakdowns are reflected in
the Absolute Maximum Ratings table. All device pins are protected with internal ESD protection diodes to the
power supplies, as shown in Figure 38.
+V_CC
External
Pin
Internal
Circuitry
− V_CC
Figure 38. Internal ESD Protection
These diodes provide moderate protection to input overdrive voltages above the supplies as well. The protection
diodes can typically support 30-mA continuous current. Use additional external low-capacitance protection where
higher currents are possible.
7.4 Device Functional Modes
7.4.1 Gain Control
The device transimpedance gain is controlled with the CTRL pin. Setting the CTRL pin high results in selecting
the high-gain configuration. Setting the CTRL pin low results in selecting the low-gain configuration, as described
in Table 1.
Table 1. Gain Control Logic Table
GAIN
4.5 kΩ
18.2 kΩ
CTRL (Pin 2)
0
1
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