Benchtop Photodiode Amplifier

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The photodiode bias voltage, monitor control output (Analog Out), and grounding jack are located on the back panel of the PDA200C.

Compatible with all of Thorlabs' Mounted Photodiodes and Biased Photodetectors.

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PDA200C Benchtop Photodiode Amplifier Connected to an SM1-Threaded Mounted Photodiode Using a BNC Cable

Features

• Transimpedance Photocurrent Amplifier
• Extremely Low Noise Operation Over Entire Dynamic Range
• 5 Digit Display with up to 10 pA Resolution
• Supports Single Point Power Calibration
• Supports Both Photodiode Polarities (CG and AG)
• Adjustable Bias Voltage
• Offset Compensation for Input Amplifier and Photodiode Dark Current
• RoHS Compliant

The PDA200C Photodiode Amplifier is ideally suited for ultra-low-noise amplification of very small photodiode currents. It offers six current ranges from 100 nA to 10 mA full scale and provides a maximum display resolution of 10 pA. The unit supports cathode grounded (CG) and anode grounded (AG) photodiodes. This amplifier may be operated in either photovoltaic or photoconductive mode. The adjustable bias voltage provides improved linearity of the responsivity and increased frequency response. This photodiode amplifier is compatible with all of Thorlabs' mounted photodiodes and biased photodetectors. Our SM05-threaded mounted photodiodes feature a female SMA connector; to convert to BNC either an adapter cable or electrical adapter may be used.

With the updated PDA200C Series, our Photocurrent Amplifier is RoHS compliant. Additionally, the current measurement ranges were changed. The rest of the features are virtually the same as the former PDA200 Series.

Thorlabs recommends recalibrating these amplifiers every 24 months and offers a factory recalibration service. To order this service, scroll to the bottom of the page and select the CAL-CCS2.

Insights into Best Lab Practices

Scroll down to read about a consideration when setting up lab equipment.

• Electrical Signals: AC vs. DC Coupling

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Electrical Signals: AC vs. DC Coupling

When an instrument offers a choice between AC and DC coupled electrical inputs, it is not unusual for the DC coupling to be the better option for a modulated input signal.

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Figure 1: The DC offset of a signal is its average value. Since the blue curve (AC Only) has an average amplitude of zero, it has a zero DC offset. The red signal (AC and DC) is identical to the blue, except the red signal has a non-zero AC offset. A DC coupling would pass the red signal unchanged. An AC coupling would remove the DC offset and attenuate low-frequency components of the signal.

AC and DC Couplings
AC and DC couplings are interfaces between the input signal and the rest of the instrument's circuitry.

A DC coupling, which is called a direct coupling, is essentially a wire connected to the signal input. This conductive coupling transmits all of the signal's frequency components, the DC as well as the AC. The red curve in Figure 1 has a non-zero DC component.

In an AC coupling, the key feature is a capacitor placed in series with the signal input. The capacitor functions as a high-pass filter and is sometimes called a blocking capacitor. AC couplings strongly attenuate the DC and low-frequency signal components. This capacitive coupling is used to remove the DC offset from the input signal, so that only AC components are passed. The blue curve in Figure 1 has only AC frequency components.

Use the DC Coupled Input When Possible
There are many reasons to prefer the DC coupled input. Its low-frequency response is very good, it allows the DC component of the signal to be monitored along with the AC, and it does not cause signal distortion since it does not affect the frequency content of the signal.

Use of the DC coupled input is recommended unless the DC offset is large or the filtering provided by the AC coupled input is required. One problem with a large DC offset is that it can reduce the resolution of the instrument to unacceptably low levels. In extreme cases, DC offsets can cause clipping and saturation effects.

Note that using the DC coupled input does not guarantee a signal free of distortion. Distortion can occur due to other reasons, such as insufficient device bandwidth or impedance mismatch at the termination.

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Figure 3: Some modulated signals, including the blue curve plotted above, have no DC component, but they do have non-negligible low-frequency components. When this signal is high-pass filtered by an AC coupling, the resulting signal is distorted. The green curve is one example of this.

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Figure 2: This frequency response magnitude plotted above models a capacitor-based high-pass filter. Its cutoff frequency (F_c) is 35 Hz, and it was used to filter the signal plotted in Figure 3. That signal has a repetition rate of 200 Hz.

Reasons to Use the AC Coupled Input
By rejecting the signal's DC component, AC coupling can reduce the total amplitude of the signal. This can increase the measurement resolution provided by the instrument, as well as overcome saturation and clipping problems. AC coupling provides good results when information is carried by high frequency signal components and low frequency components are not of interest. AC coupling can also be preferred when the application does not tolerate DC frequency signal components, as is the case for some telecommunications applications.

When Using the AC Coupled Input
If AC coupling is used, it is important to keep in mind that this coupling acts as a high pass filter and affects the frequency content of the signal.

As illustrated by Figure 2, this coupling does not just remove the DC offset, it can also attenuate low frequency components that may be of interest. Due to this, AC coupling can result in signal distortion. To illustrate the effects of high-pass filtering, Figure 3 plots a binary signal, with 200 Hz repetition rate, before and after it is filtered by the high-pass filter with 35 Hz cutoff frequency (F_c).

AC-coupled, digital telecommunications signals mitigate this problem by ensuring the signals are DC balanced, so that they have no DC offset. If the signals were not DC balanced, a series of ones could cause a sustained high signal level. This would introduce a non-zero DC level that would cause the signal to be affected by the capacitive filtering. The result could be bit errors due to high states being incorrectly read as low states.

Date of Last Edit: Dec. 4, 2019

Transimpedance Amplifier Selection Guide

Representative Photoa	Item #	Zero Offset	PD Bias Voltage	Bandwidth (3 dB)	Transimpedance Gain	Max Input Current	Max Output Voltage	Max PD Capacitanceb	Power Supply	Mass	Dimensions
Fixed Gain, Compact
	AMP110		-	1 kHz	107 V/A	±200 nA	±2 V	10 nF	USB, 5 V @ 2 A	80 g	97 mm x 32 mm x 25.4 mm
	AMP120		-	100 kHz	105 V/A	±20 µA	±2 V	10 nF
	AMP130		-	100 kHz	103 V/A	±2 mA	±2 V	10 nF
	AMP140	-		10 MHz	104 V/A	±350 µA	±3.5 Vc	200 pF			96 mm x 32 mm x 25.4 mm
Switchable Gain, Compact
	AMP100		-	1 kHz	106 V/A	±2 µA	±2 V	10 nF	USB, 5 V @ 2 A	80 g	97 mm x 32 mm x 25.4 mm
					107 V/A	±200 nA
					108 V/A	±20 nA
	AMP102		-	100 kHz	103 V/A	±2 mA	±2 V	10 nF
					104 V/A	±200 µA
					105 V/A	±20 µA
Switchable Gain, Benchtop with 5-Digit LED Display
	PDA200C			1 kHz	108 V/A	+100 nA	±10 Vd	10 nF	100 V, 115 V, or 230 V (50/60 Hz)	<3 kg	320 mm x 146 mm x 77 mm
				5 kHz	107 V/A	+1 µA
				20 kHz	106 V/A	+10 µA
				70 kHz	105 V/A	+100 µA
				250 kHz	104 V/A	+1 mA
				500 kHz	103 V/A	+10 mA

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• Maximum Photodiode Capacitance Required for Frequency Compensated Operation
• Bias Voltage: +1.5 V to +15 V (CG Polarity), -1.5 V to -15 V (AG Polarity)
• Output Voltage Ranges: 0 to +10 V (CG), 0 to -10 V (AG); Bias Adjustment: 0 V to -10 V (CG), 0 V to +10 V (AG)