OPA1662-Q1: P2P Sulution to Replace NJM4558M

Hi Fery,

I'd recommend OPA1662-Q1. One concern here is that if the customer is using the 'M' variant of the NJM4558, that variant comes in what NJR calls a DMP-8 package, which is a wider version of the SOIC we offer the OPA1662-Q1 in. Depending on the customer's PCB layout, it may be possible to directly substitute the OPA1662-Q1, but this swap may require a change to the footprint in the customer's design. If the customer is using the 'E' variant of the NJM4558 it is an SOIC package, and can be directly substituted.

For single-ended to differential conversion, the closest automotive-qualified solution we have would be to use either an automotive-qualified op amp (such as the OPA1662-Q1) to implement the single-ended to differential converter circuit shown on page 86 of the Analog Engineer's Circuit Cookbook, or to use an INA1650-Q1 configured as shown in Figure 62 of the INA1650 Datasheet. 

Hi David,

We have doubled check with customer, they are using NJM4558E, with package of SOP8, and LM2904-Q1 should be P2P with NJM4558E.

Could you please help to check below SCH, customer is intended to use LM2904-Q1 to convert single ended audio signal to differential out.

Hi Fery,

With 0 ohm resistors and 1nF capacitors directly on the LM2904 outputs, I suspect that this circuit will be unstable. If the customer needs the 1nF capacitors for EMI protection or additional filtering, I'd recommend replacing R1023 and R1024 with 10-100ohm resistors. 

As drawn, this circuit won't perform as intended. Right now, channel 1 of the LM2904 will attempt to output 0V, since the DC bias on its input (at pin 3) is connected to GND. This will then, in turn, cause channel 2 of the op amp to slam into its positive rail. This can be fixed by connecting R1002 to AVAS_OP_VREF instead of GND. 

I am unclear as to the purpose of R1001 in this circuit. If this was located between C870 and R1002 it would adjust the gain of the single-ended to differential conversion so that the differential output voltage would be equal to the voltage at AVAS_AUDIO. As drawn right now, R1001 will interact with the input capacitance of U110 to produce an additional low-pass filter (though out at significantly higher frequencies).

Hi Fery,

I agree with Alexander, there are some mistakes in your schematic. I would do it this way:

fery.TSC

Kai

Hi Goeke,

A follow up question on this ticket, customer are using below circuit for their reference to convert singled ended signal to differential signal.

But on below circuit, the output signal level is equal to the input signal level. We wondering how to configure the gain below output and input? Or we have other solution to convert single ended signal to differential signal, and amplify the signal at the same time?

Thanks!

Hi Fery,

I'd recommend the following circuit as a starting point:

In order to provide gain, U1 must be configured as a noninverting amplifier. To maintain the proper DC bias at U1's output, however, C3 has to be added to the feedback network. This decouples the feedback network at DC and low frequencies, ensuring that DC gain is unity. In this example, R4 and R5 create an AC gain of ~5x for the input signal, resulting in a 10x gain at the differential output. 

Since R4 and C3 interact to create a high-pass corner in the op amp gain, in this case around 1.5Hz. Changes to R4 will affect this cutoff frequency. AC simulations indicated that this design provided a gain of 20dB, flat to within .1dB from ~17Hz to ~30kHz.